[{"data":1,"prerenderedAt":3146},["ShallowReactive",2],{"page-\u002Fcli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002Fstreaming-subprocess-output-in-real-time\u002F":3,"content-directory":2301},{"id":4,"title":5,"body":6,"date":2286,"description":2287,"difficulty":2288,"draft":2289,"extension":2290,"meta":2291,"navigation":99,"path":2292,"seo":2293,"stem":2294,"tags":2295,"updated":2286,"__hash__":2300},"content\u002Fcli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002Fstreaming-subprocess-output-in-real-time\u002Findex.md","Streaming Subprocess Output in Real Time",{"type":7,"value":8,"toc":2269},"minimark",[9,28,33,50,57,213,217,233,237,672,675,1144,1151,1170,1174,1182,1185,1188,1191,1248,1251,1255,1262,1292,1295,1298,1640,1666,1670,1737,1741,1748,2131,2142,2146,2160,2164,2173,2182,2186,2201,2205,2219,2223,2231,2235,2265],[10,11,12,13,17,18,21,22,27],"p",{},"Your CLI runs a build, a test suite or a migration that takes minutes, and with ",[14,15,16],"code",{},"subprocess.run(capture_output=True)"," the user stares at a silent terminal until it finishes — then gets the entire log at once. They want to see each line as the child produces it, ideally prefixed with which step it came from, while your tool still keeps a copy to inspect afterwards. This guide shows how to stream output from a child process line by line with ",[14,19,20],{},"Popen",", why output sometimes still arrives in bursts even when you read it correctly, and how to handle stdout and stderr separately without deadlocking. It builds on the safe-call helper from ",[23,24,26],"a",{"href":25},"\u002Fcli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002Fcalling-external-commands-safely-with-subprocess\u002F","calling external commands safely with subprocess",".",[29,30,32],"h2",{"id":31},"prerequisites","Prerequisites",[34,35,36,40,47],"ul",{},[37,38,39],"li",{},"Python 3.10+ and a Typer or Click CLI.",[37,41,42,43,46],{},"Familiarity with ",[14,44,45],{},"subprocess.run()"," and its keyword arguments.",[37,48,49],{},"A long-running command to try it on. The examples use a tiny Python script as the child so you can reproduce everything without extra tools.",[10,51,52,53,56],{},"Here is that child. Save it as ",[14,54,55],{},"slow.py","; it prints a line every half second, with a warning on stderr in the middle:",[58,59,64],"pre",{"className":60,"code":61,"language":62,"meta":63,"style":63},"language-python shiki shiki-themes github-light github-dark","# slow.py — a stand-in for a build or test run\nimport sys\nimport time\n\nfor i in range(1, 7):\n    print(f\"step {i}\u002F6 done\")\n    if i == 3:\n        print(\"warning: cache miss, rebuilding\", file=sys.stderr)\n    time.sleep(0.5)\n","python","",[14,65,66,75,86,94,101,132,162,179,202],{"__ignoreMap":63},[67,68,71],"span",{"class":69,"line":70},"line",1,[67,72,74],{"class":73},"sJ8bj","# slow.py — a stand-in for a build or test run\n",[67,76,78,82],{"class":69,"line":77},2,[67,79,81],{"class":80},"szBVR","import",[67,83,85],{"class":84},"sVt8B"," sys\n",[67,87,89,91],{"class":69,"line":88},3,[67,90,81],{"class":80},[67,92,93],{"class":84}," time\n",[67,95,97],{"class":69,"line":96},4,[67,98,100],{"emptyLinePlaceholder":99},true,"\n",[67,102,104,107,110,113,117,120,123,126,129],{"class":69,"line":103},5,[67,105,106],{"class":80},"for",[67,108,109],{"class":84}," i ",[67,111,112],{"class":80},"in",[67,114,116],{"class":115},"sj4cs"," range",[67,118,119],{"class":84},"(",[67,121,122],{"class":115},"1",[67,124,125],{"class":84},", ",[67,127,128],{"class":115},"7",[67,130,131],{"class":84},"):\n",[67,133,135,138,140,143,147,150,153,156,159],{"class":69,"line":134},6,[67,136,137],{"class":115},"    print",[67,139,119],{"class":84},[67,141,142],{"class":80},"f",[67,144,146],{"class":145},"sZZnC","\"step ",[67,148,149],{"class":115},"{",[67,151,152],{"class":84},"i",[67,154,155],{"class":115},"}",[67,157,158],{"class":145},"\u002F6 done\"",[67,160,161],{"class":84},")\n",[67,163,165,168,170,173,176],{"class":69,"line":164},7,[67,166,167],{"class":80},"    if",[67,169,109],{"class":84},[67,171,172],{"class":80},"==",[67,174,175],{"class":115}," 3",[67,177,178],{"class":84},":\n",[67,180,182,185,187,190,192,196,199],{"class":69,"line":181},8,[67,183,184],{"class":115},"        print",[67,186,119],{"class":84},[67,188,189],{"class":145},"\"warning: cache miss, rebuilding\"",[67,191,125],{"class":84},[67,193,195],{"class":194},"s4XuR","file",[67,197,198],{"class":80},"=",[67,200,201],{"class":84},"sys.stderr)\n",[67,203,205,208,211],{"class":69,"line":204},9,[67,206,207],{"class":84},"    time.sleep(",[67,209,210],{"class":115},"0.5",[67,212,161],{"class":84},[29,214,216],{"id":215},"the-recipe-iterate-over-the-pipe","The recipe: iterate over the pipe",[10,218,219,221,222,225,226,125,229,232],{},[14,220,20],{}," starts the child and returns immediately. With ",[14,223,224],{},"stdout=subprocess.PIPE"," and ",[14,227,228],{},"text=True",[14,230,231],{},"proc.stdout"," is a file object you can iterate; each iteration blocks until the child has written a complete line, then hands it to you. Nothing is accumulated unless you choose to keep it.",[234,235],"inline-diagram",{"name":236},"sp-stream-pipeline",[58,238,240],{"className":60,"code":239,"language":62,"meta":63,"style":63},"# src\u002Fmytool\u002Fstream.py\nfrom __future__ import annotations\n\nimport subprocess\nimport sys\nfrom collections import deque\nfrom collections.abc import Callable\n\n\ndef stream_command(\n    argv: list[str],\n    *,\n    on_line: Callable[[str], None],\n    keep_last: int = 50,\n    env: dict[str, str] | None = None,\n) -> tuple[int, list[str]]:\n    \"\"\"Run argv, call on_line for each output line, return (code, tail).\"\"\"\n    tail: deque[str] = deque(maxlen=keep_last)\n    with subprocess.Popen(\n        argv,\n        stdout=subprocess.PIPE,\n        stderr=subprocess.STDOUT,      # one ordered stream\n        stdin=subprocess.DEVNULL,\n        text=True,\n        encoding=\"utf-8\",\n        errors=\"replace\",\n        bufsize=1,                     # line-buffered on our side\n        env=env,\n    ) as proc:\n        assert proc.stdout is not None\n        for raw in proc.stdout:\n            line = raw.rstrip(\"\\n\")\n            tail.append(line)\n            on_line(line)\n    return proc.returncode, list(tail)\n",[14,241,242,247,261,265,272,278,290,302,306,310,323,335,344,360,377,404,420,426,449,458,464,480,499,514,527,540,553,569,580,592,610,624,645,651,657],{"__ignoreMap":63},[67,243,244],{"class":69,"line":70},[67,245,246],{"class":73},"# src\u002Fmytool\u002Fstream.py\n",[67,248,249,252,255,258],{"class":69,"line":77},[67,250,251],{"class":80},"from",[67,253,254],{"class":115}," __future__",[67,256,257],{"class":80}," import",[67,259,260],{"class":84}," annotations\n",[67,262,263],{"class":69,"line":88},[67,264,100],{"emptyLinePlaceholder":99},[67,266,267,269],{"class":69,"line":96},[67,268,81],{"class":80},[67,270,271],{"class":84}," subprocess\n",[67,273,274,276],{"class":69,"line":103},[67,275,81],{"class":80},[67,277,85],{"class":84},[67,279,280,282,285,287],{"class":69,"line":134},[67,281,251],{"class":80},[67,283,284],{"class":84}," collections ",[67,286,81],{"class":80},[67,288,289],{"class":84}," deque\n",[67,291,292,294,297,299],{"class":69,"line":164},[67,293,251],{"class":80},[67,295,296],{"class":84}," collections.abc ",[67,298,81],{"class":80},[67,300,301],{"class":84}," Callable\n",[67,303,304],{"class":69,"line":181},[67,305,100],{"emptyLinePlaceholder":99},[67,307,308],{"class":69,"line":204},[67,309,100],{"emptyLinePlaceholder":99},[67,311,313,316,320],{"class":69,"line":312},10,[67,314,315],{"class":80},"def",[67,317,319],{"class":318},"sScJk"," stream_command",[67,321,322],{"class":84},"(\n",[67,324,326,329,332],{"class":69,"line":325},11,[67,327,328],{"class":84},"    argv: list[",[67,330,331],{"class":115},"str",[67,333,334],{"class":84},"],\n",[67,336,338,341],{"class":69,"line":337},12,[67,339,340],{"class":80},"    *",[67,342,343],{"class":84},",\n",[67,345,347,350,352,355,358],{"class":69,"line":346},13,[67,348,349],{"class":84},"    on_line: Callable[[",[67,351,331],{"class":115},[67,353,354],{"class":84},"], ",[67,356,357],{"class":115},"None",[67,359,334],{"class":84},[67,361,363,366,369,372,375],{"class":69,"line":362},14,[67,364,365],{"class":84},"    keep_last: ",[67,367,368],{"class":115},"int",[67,370,371],{"class":80}," =",[67,373,374],{"class":115}," 50",[67,376,343],{"class":84},[67,378,380,383,385,387,389,392,395,398,400,402],{"class":69,"line":379},15,[67,381,382],{"class":84},"    env: dict[",[67,384,331],{"class":115},[67,386,125],{"class":84},[67,388,331],{"class":115},[67,390,391],{"class":84},"] ",[67,393,394],{"class":80},"|",[67,396,397],{"class":115}," None",[67,399,371],{"class":80},[67,401,397],{"class":115},[67,403,343],{"class":84},[67,405,407,410,412,415,417],{"class":69,"line":406},16,[67,408,409],{"class":84},") -> tuple[",[67,411,368],{"class":115},[67,413,414],{"class":84},", list[",[67,416,331],{"class":115},[67,418,419],{"class":84},"]]:\n",[67,421,423],{"class":69,"line":422},17,[67,424,425],{"class":145},"    \"\"\"Run argv, call on_line for each output line, return (code, tail).\"\"\"\n",[67,427,429,432,434,436,438,441,444,446],{"class":69,"line":428},18,[67,430,431],{"class":84},"    tail: deque[",[67,433,331],{"class":115},[67,435,391],{"class":84},[67,437,198],{"class":80},[67,439,440],{"class":84}," deque(",[67,442,443],{"class":194},"maxlen",[67,445,198],{"class":80},[67,447,448],{"class":84},"keep_last)\n",[67,450,452,455],{"class":69,"line":451},19,[67,453,454],{"class":80},"    with",[67,456,457],{"class":84}," subprocess.Popen(\n",[67,459,461],{"class":69,"line":460},20,[67,462,463],{"class":84},"        argv,\n",[67,465,467,470,472,475,478],{"class":69,"line":466},21,[67,468,469],{"class":194},"        stdout",[67,471,198],{"class":80},[67,473,474],{"class":84},"subprocess.",[67,476,477],{"class":115},"PIPE",[67,479,343],{"class":84},[67,481,483,486,488,490,493,496],{"class":69,"line":482},22,[67,484,485],{"class":194},"        stderr",[67,487,198],{"class":80},[67,489,474],{"class":84},[67,491,492],{"class":115},"STDOUT",[67,494,495],{"class":84},",      ",[67,497,498],{"class":73},"# one ordered stream\n",[67,500,502,505,507,509,512],{"class":69,"line":501},23,[67,503,504],{"class":194},"        stdin",[67,506,198],{"class":80},[67,508,474],{"class":84},[67,510,511],{"class":115},"DEVNULL",[67,513,343],{"class":84},[67,515,517,520,522,525],{"class":69,"line":516},24,[67,518,519],{"class":194},"        text",[67,521,198],{"class":80},[67,523,524],{"class":115},"True",[67,526,343],{"class":84},[67,528,530,533,535,538],{"class":69,"line":529},25,[67,531,532],{"class":194},"        encoding",[67,534,198],{"class":80},[67,536,537],{"class":145},"\"utf-8\"",[67,539,343],{"class":84},[67,541,543,546,548,551],{"class":69,"line":542},26,[67,544,545],{"class":194},"        errors",[67,547,198],{"class":80},[67,549,550],{"class":145},"\"replace\"",[67,552,343],{"class":84},[67,554,556,559,561,563,566],{"class":69,"line":555},27,[67,557,558],{"class":194},"        bufsize",[67,560,198],{"class":80},[67,562,122],{"class":115},[67,564,565],{"class":84},",                     ",[67,567,568],{"class":73},"# line-buffered on our side\n",[67,570,572,575,577],{"class":69,"line":571},28,[67,573,574],{"class":194},"        env",[67,576,198],{"class":80},[67,578,579],{"class":84},"env,\n",[67,581,583,586,589],{"class":69,"line":582},29,[67,584,585],{"class":84},"    ) ",[67,587,588],{"class":80},"as",[67,590,591],{"class":84}," proc:\n",[67,593,595,598,601,604,607],{"class":69,"line":594},30,[67,596,597],{"class":80},"        assert",[67,599,600],{"class":84}," proc.stdout ",[67,602,603],{"class":80},"is",[67,605,606],{"class":80}," not",[67,608,609],{"class":115}," None\n",[67,611,613,616,619,621],{"class":69,"line":612},31,[67,614,615],{"class":80},"        for",[67,617,618],{"class":84}," raw ",[67,620,112],{"class":80},[67,622,623],{"class":84}," proc.stdout:\n",[67,625,627,630,632,635,638,641,643],{"class":69,"line":626},32,[67,628,629],{"class":84},"            line ",[67,631,198],{"class":80},[67,633,634],{"class":84}," raw.rstrip(",[67,636,637],{"class":145},"\"",[67,639,640],{"class":115},"\\n",[67,642,637],{"class":145},[67,644,161],{"class":84},[67,646,648],{"class":69,"line":647},33,[67,649,650],{"class":84},"            tail.append(line)\n",[67,652,654],{"class":69,"line":653},34,[67,655,656],{"class":84},"            on_line(line)\n",[67,658,660,663,666,669],{"class":69,"line":659},35,[67,661,662],{"class":80},"    return",[67,664,665],{"class":84}," proc.returncode, ",[67,667,668],{"class":115},"list",[67,670,671],{"class":84},"(tail)\n",[10,673,674],{},"And the command that uses it:",[58,676,678],{"className":60,"code":677,"language":62,"meta":63,"style":63},"# src\u002Fmytool\u002Fcli.py\nimport os\nimport sys\n\nimport typer\n\nfrom mytool.stream import stream_command\n\napp = typer.Typer()\n\n\n@app.callback()\ndef main() -> None:\n    \"\"\"Build helpers.\"\"\"\n\n\n@app.command()\ndef build(verbose: bool = typer.Option(False, \"--verbose\", \"-v\")) -> None:\n    \"\"\"Run the build and show its progress.\"\"\"\n    env = {**os.environ, \"PYTHONUNBUFFERED\": \"1\"}\n\n    def show(line: str) -> None:\n        if verbose or line.startswith((\"step\", \"warning\")):\n            typer.echo(f\"  build │ {line}\", err=True)\n\n    code, tail = stream_command([sys.executable, \"slow.py\"], on_line=show, env=env)\n    if code != 0:\n        typer.secho(f\"build failed (exit {code}); last lines:\", fg=\"red\", err=True)\n        for line in tail[-10:]:\n            typer.echo(f\"  {line}\", err=True)\n        raise typer.Exit(1)\n    typer.secho(\"build finished\", fg=\"green\", err=True)\n\n\nif __name__ == \"__main__\":\n    app()\n",[14,679,680,685,692,698,702,709,713,725,729,739,743,747,755,769,774,778,782,789,827,832,860,864,884,909,938,942,973,988,1027,1048,1075,1087,1114,1118,1122,1138],{"__ignoreMap":63},[67,681,682],{"class":69,"line":70},[67,683,684],{"class":73},"# src\u002Fmytool\u002Fcli.py\n",[67,686,687,689],{"class":69,"line":77},[67,688,81],{"class":80},[67,690,691],{"class":84}," os\n",[67,693,694,696],{"class":69,"line":88},[67,695,81],{"class":80},[67,697,85],{"class":84},[67,699,700],{"class":69,"line":96},[67,701,100],{"emptyLinePlaceholder":99},[67,703,704,706],{"class":69,"line":103},[67,705,81],{"class":80},[67,707,708],{"class":84}," typer\n",[67,710,711],{"class":69,"line":134},[67,712,100],{"emptyLinePlaceholder":99},[67,714,715,717,720,722],{"class":69,"line":164},[67,716,251],{"class":80},[67,718,719],{"class":84}," mytool.stream ",[67,721,81],{"class":80},[67,723,724],{"class":84}," stream_command\n",[67,726,727],{"class":69,"line":181},[67,728,100],{"emptyLinePlaceholder":99},[67,730,731,734,736],{"class":69,"line":204},[67,732,733],{"class":84},"app ",[67,735,198],{"class":80},[67,737,738],{"class":84}," typer.Typer()\n",[67,740,741],{"class":69,"line":312},[67,742,100],{"emptyLinePlaceholder":99},[67,744,745],{"class":69,"line":325},[67,746,100],{"emptyLinePlaceholder":99},[67,748,749,752],{"class":69,"line":337},[67,750,751],{"class":318},"@app.callback",[67,753,754],{"class":84},"()\n",[67,756,757,759,762,765,767],{"class":69,"line":346},[67,758,315],{"class":80},[67,760,761],{"class":318}," main",[67,763,764],{"class":84},"() -> ",[67,766,357],{"class":115},[67,768,178],{"class":84},[67,770,771],{"class":69,"line":362},[67,772,773],{"class":145},"    \"\"\"Build helpers.\"\"\"\n",[67,775,776],{"class":69,"line":379},[67,777,100],{"emptyLinePlaceholder":99},[67,779,780],{"class":69,"line":406},[67,781,100],{"emptyLinePlaceholder":99},[67,783,784,787],{"class":69,"line":422},[67,785,786],{"class":318},"@app.command",[67,788,754],{"class":84},[67,790,791,793,796,799,802,804,807,810,812,815,817,820,823,825],{"class":69,"line":428},[67,792,315],{"class":80},[67,794,795],{"class":318}," build",[67,797,798],{"class":84},"(verbose: ",[67,800,801],{"class":115},"bool",[67,803,371],{"class":80},[67,805,806],{"class":84}," typer.Option(",[67,808,809],{"class":115},"False",[67,811,125],{"class":84},[67,813,814],{"class":145},"\"--verbose\"",[67,816,125],{"class":84},[67,818,819],{"class":145},"\"-v\"",[67,821,822],{"class":84},")) -> ",[67,824,357],{"class":115},[67,826,178],{"class":84},[67,828,829],{"class":69,"line":451},[67,830,831],{"class":145},"    \"\"\"Run the build and show its progress.\"\"\"\n",[67,833,834,837,839,842,845,848,851,854,857],{"class":69,"line":460},[67,835,836],{"class":84},"    env ",[67,838,198],{"class":80},[67,840,841],{"class":84}," {",[67,843,844],{"class":80},"**",[67,846,847],{"class":84},"os.environ, ",[67,849,850],{"class":145},"\"PYTHONUNBUFFERED\"",[67,852,853],{"class":84},": ",[67,855,856],{"class":145},"\"1\"",[67,858,859],{"class":84},"}\n",[67,861,862],{"class":69,"line":466},[67,863,100],{"emptyLinePlaceholder":99},[67,865,866,869,872,875,877,880,882],{"class":69,"line":482},[67,867,868],{"class":80},"    def",[67,870,871],{"class":318}," show",[67,873,874],{"class":84},"(line: ",[67,876,331],{"class":115},[67,878,879],{"class":84},") -> ",[67,881,357],{"class":115},[67,883,178],{"class":84},[67,885,886,889,892,895,898,901,903,906],{"class":69,"line":501},[67,887,888],{"class":80},"        if",[67,890,891],{"class":84}," verbose ",[67,893,894],{"class":80},"or",[67,896,897],{"class":84}," line.startswith((",[67,899,900],{"class":145},"\"step\"",[67,902,125],{"class":84},[67,904,905],{"class":145},"\"warning\"",[67,907,908],{"class":84},")):\n",[67,910,911,914,916,919,921,923,925,927,929,932,934,936],{"class":69,"line":516},[67,912,913],{"class":84},"            typer.echo(",[67,915,142],{"class":80},[67,917,918],{"class":145},"\"  build │ ",[67,920,149],{"class":115},[67,922,69],{"class":84},[67,924,155],{"class":115},[67,926,637],{"class":145},[67,928,125],{"class":84},[67,930,931],{"class":194},"err",[67,933,198],{"class":80},[67,935,524],{"class":115},[67,937,161],{"class":84},[67,939,940],{"class":69,"line":529},[67,941,100],{"emptyLinePlaceholder":99},[67,943,944,947,949,952,955,957,960,962,965,968,970],{"class":69,"line":542},[67,945,946],{"class":84},"    code, tail ",[67,948,198],{"class":80},[67,950,951],{"class":84}," stream_command([sys.executable, ",[67,953,954],{"class":145},"\"slow.py\"",[67,956,354],{"class":84},[67,958,959],{"class":194},"on_line",[67,961,198],{"class":80},[67,963,964],{"class":84},"show, ",[67,966,967],{"class":194},"env",[67,969,198],{"class":80},[67,971,972],{"class":84},"env)\n",[67,974,975,977,980,983,986],{"class":69,"line":555},[67,976,167],{"class":80},[67,978,979],{"class":84}," code ",[67,981,982],{"class":80},"!=",[67,984,985],{"class":115}," 0",[67,987,178],{"class":84},[67,989,990,993,995,998,1000,1002,1004,1007,1009,1012,1014,1017,1019,1021,1023,1025],{"class":69,"line":571},[67,991,992],{"class":84},"        typer.secho(",[67,994,142],{"class":80},[67,996,997],{"class":145},"\"build failed (exit ",[67,999,149],{"class":115},[67,1001,14],{"class":84},[67,1003,155],{"class":115},[67,1005,1006],{"class":145},"); last lines:\"",[67,1008,125],{"class":84},[67,1010,1011],{"class":194},"fg",[67,1013,198],{"class":80},[67,1015,1016],{"class":145},"\"red\"",[67,1018,125],{"class":84},[67,1020,931],{"class":194},[67,1022,198],{"class":80},[67,1024,524],{"class":115},[67,1026,161],{"class":84},[67,1028,1029,1031,1034,1036,1039,1042,1045],{"class":69,"line":582},[67,1030,615],{"class":80},[67,1032,1033],{"class":84}," line ",[67,1035,112],{"class":80},[67,1037,1038],{"class":84}," tail[",[67,1040,1041],{"class":80},"-",[67,1043,1044],{"class":115},"10",[67,1046,1047],{"class":84},":]:\n",[67,1049,1050,1052,1054,1057,1059,1061,1063,1065,1067,1069,1071,1073],{"class":69,"line":594},[67,1051,913],{"class":84},[67,1053,142],{"class":80},[67,1055,1056],{"class":145},"\"  ",[67,1058,149],{"class":115},[67,1060,69],{"class":84},[67,1062,155],{"class":115},[67,1064,637],{"class":145},[67,1066,125],{"class":84},[67,1068,931],{"class":194},[67,1070,198],{"class":80},[67,1072,524],{"class":115},[67,1074,161],{"class":84},[67,1076,1077,1080,1083,1085],{"class":69,"line":612},[67,1078,1079],{"class":80},"        raise",[67,1081,1082],{"class":84}," typer.Exit(",[67,1084,122],{"class":115},[67,1086,161],{"class":84},[67,1088,1089,1092,1095,1097,1099,1101,1104,1106,1108,1110,1112],{"class":69,"line":626},[67,1090,1091],{"class":84},"    typer.secho(",[67,1093,1094],{"class":145},"\"build finished\"",[67,1096,125],{"class":84},[67,1098,1011],{"class":194},[67,1100,198],{"class":80},[67,1102,1103],{"class":145},"\"green\"",[67,1105,125],{"class":84},[67,1107,931],{"class":194},[67,1109,198],{"class":80},[67,1111,524],{"class":115},[67,1113,161],{"class":84},[67,1115,1116],{"class":69,"line":647},[67,1117,100],{"emptyLinePlaceholder":99},[67,1119,1120],{"class":69,"line":653},[67,1121,100],{"emptyLinePlaceholder":99},[67,1123,1124,1127,1130,1133,1136],{"class":69,"line":659},[67,1125,1126],{"class":80},"if",[67,1128,1129],{"class":115}," __name__",[67,1131,1132],{"class":80}," ==",[67,1134,1135],{"class":145}," \"__main__\"",[67,1137,178],{"class":84},[67,1139,1141],{"class":69,"line":1140},36,[67,1142,1143],{"class":84},"    app()\n",[10,1145,1146,1147,1150],{},"The ",[14,1148,1149],{},"deque(maxlen=...)"," is the important memory decision: you keep the last fifty lines for the failure report, not the whole log. A build that prints for an hour costs the same memory as one that prints for a second.",[10,1152,1146,1153,1156,1157,1160,1161,1164,1165,1169],{},[14,1154,1155],{},"with"," block matters too. When the loop ends because the child closed its stdout, leaving the block waits for the process and sets ",[14,1158,1159],{},"returncode",". If an exception is raised inside the loop — a ",[14,1162,1163],{},"KeyboardInterrupt",", say — the context manager still closes the pipe and waits, so no zombie is left behind. Pair it with the approach in ",[23,1166,1168],{"href":1167},"\u002Fadvanced-input-parsing-user-experience\u002Ferror-handling-and-exit-codes\u002Fhandling-keyboard-interrupt-cleanly\u002F","handling KeyboardInterrupt cleanly"," if the child should be stopped too.",[29,1171,1173],{"id":1172},"why-output-still-arrives-in-bursts","Why output still arrives in bursts",[10,1175,1176,1177,1181],{},"You run the code above against a real program and the lines appear twenty at a time. Your loop is not the problem. The ",[1178,1179,1180],"strong",{},"child"," is buffering.",[10,1183,1184],{},"Most programs — Python, anything using C's stdio, many Go and Rust tools — check whether their stdout is a terminal. If it is, they flush after every line so a person sees output promptly. If it is a pipe, they switch to block buffering and flush only when a buffer of several kilobytes fills, because that is much faster for bulk data. Your CLI reads from a pipe, so it gets blocks.",[234,1186],{"name":1187},"sp-buffering-matrix",[10,1189,1190],{},"The fix belongs on the child's side, and it depends on the child:",[34,1192,1193,1210,1220,1238],{},[37,1194,1195,1198,1199,1202,1203,1206,1207,1209],{},[1178,1196,1197],{},"Python children:"," set ",[14,1200,1201],{},"PYTHONUNBUFFERED=1"," in the environment, or run with ",[14,1204,1205],{},"python -u",". That is why the command above builds ",[14,1208,967],{}," with it.",[37,1211,1212,1215,1216,1219],{},[1178,1213,1214],{},"C programs using stdio:"," on Linux and macOS with GNU coreutils, prefix the command with ",[14,1217,1218],{},"stdbuf -oL"," to force line buffering.",[37,1221,1222,1225,1226,125,1229,125,1232,125,1235,27],{},[1178,1223,1224],{},"Tools with their own flag:"," many have one — ",[14,1227,1228],{},"grep --line-buffered",[14,1230,1231],{},"sed -u",[14,1233,1234],{},"jq --unbuffered",[14,1236,1237],{},"docker build --progress=plain",[37,1239,1240,1243,1244,1247],{},[1178,1241,1242],{},"Programs that only behave on a terminal:"," run them under a pseudo-terminal with the standard-library ",[14,1245,1246],{},"pty"," module (POSIX only). The child then believes it is talking to a person, colour codes and all, which you may need to strip.",[10,1249,1250],{},"Reach for the pseudo-terminal last. It changes more than buffering — the child may start prompting, drawing progress bars with carriage returns, or emitting escape codes — and it does not exist on Windows.",[29,1252,1254],{"id":1253},"two-streams-without-a-deadlock","Two streams without a deadlock",[10,1256,1257,1258,1261],{},"Merging stderr into stdout (",[14,1259,1260],{},"stderr=subprocess.STDOUT",") gives one ordered stream and is the right default for a live log. But sometimes you need them apart: you parse stdout as data and show stderr as diagnostics. The naive version reads one pipe, then the other:",[58,1263,1265],{"className":60,"code":1264,"language":62,"meta":63,"style":63},"# DON'T: can deadlock\nout = proc.stdout.read()\nerr = proc.stderr.read()\n",[14,1266,1267,1272,1282],{"__ignoreMap":63},[67,1268,1269],{"class":69,"line":70},[67,1270,1271],{"class":73},"# DON'T: can deadlock\n",[67,1273,1274,1277,1279],{"class":69,"line":77},[67,1275,1276],{"class":84},"out ",[67,1278,198],{"class":80},[67,1280,1281],{"class":84}," proc.stdout.read()\n",[67,1283,1284,1287,1289],{"class":69,"line":88},[67,1285,1286],{"class":84},"err ",[67,1288,198],{"class":80},[67,1290,1291],{"class":84}," proc.stderr.read()\n",[10,1293,1294],{},"If the child writes more to stderr than the pipe buffer holds (64 KB on Linux) while you are blocked reading stdout, the child blocks on its write, you block on your read, and neither moves. The fix is to read both concurrently. Threads are the simplest portable way:",[234,1296],{"name":1297},"sp-merge-streams",[58,1299,1301],{"className":60,"code":1300,"language":62,"meta":63,"style":63},"import subprocess\nimport threading\nfrom collections.abc import Callable\nfrom typing import IO\n\n\ndef _pump(stream: IO[str], handle: Callable[[str], None]) -> None:\n    for line in stream:\n        handle(line.rstrip(\"\\n\"))\n\n\ndef stream_two(argv: list[str], on_out: Callable[[str], None],\n               on_err: Callable[[str], None]) -> int:\n    with subprocess.Popen(argv, stdout=subprocess.PIPE, stderr=subprocess.PIPE,\n                          stdin=subprocess.DEVNULL, text=True, encoding=\"utf-8\",\n                          errors=\"replace\") as proc:\n        threads = [\n            threading.Thread(target=_pump, args=(proc.stdout, on_out), daemon=True),\n            threading.Thread(target=_pump, args=(proc.stderr, on_err), daemon=True),\n        ]\n        for t in threads:\n            t.start()\n        for t in threads:\n            t.join()\n    return proc.returncode\n",[14,1302,1303,1309,1316,1326,1338,1342,1346,1380,1392,1406,1410,1414,1437,1454,1483,1514,1530,1540,1571,1596,1601,1613,1618,1628,1633],{"__ignoreMap":63},[67,1304,1305,1307],{"class":69,"line":70},[67,1306,81],{"class":80},[67,1308,271],{"class":84},[67,1310,1311,1313],{"class":69,"line":77},[67,1312,81],{"class":80},[67,1314,1315],{"class":84}," threading\n",[67,1317,1318,1320,1322,1324],{"class":69,"line":88},[67,1319,251],{"class":80},[67,1321,296],{"class":84},[67,1323,81],{"class":80},[67,1325,301],{"class":84},[67,1327,1328,1330,1333,1335],{"class":69,"line":96},[67,1329,251],{"class":80},[67,1331,1332],{"class":84}," typing ",[67,1334,81],{"class":80},[67,1336,1337],{"class":115}," IO\n",[67,1339,1340],{"class":69,"line":103},[67,1341,100],{"emptyLinePlaceholder":99},[67,1343,1344],{"class":69,"line":134},[67,1345,100],{"emptyLinePlaceholder":99},[67,1347,1348,1350,1353,1356,1359,1362,1364,1367,1369,1371,1373,1376,1378],{"class":69,"line":164},[67,1349,315],{"class":80},[67,1351,1352],{"class":318}," _pump",[67,1354,1355],{"class":84},"(stream: ",[67,1357,1358],{"class":115},"IO",[67,1360,1361],{"class":84},"[",[67,1363,331],{"class":115},[67,1365,1366],{"class":84},"], handle: Callable[[",[67,1368,331],{"class":115},[67,1370,354],{"class":84},[67,1372,357],{"class":115},[67,1374,1375],{"class":84},"]) -> ",[67,1377,357],{"class":115},[67,1379,178],{"class":84},[67,1381,1382,1385,1387,1389],{"class":69,"line":181},[67,1383,1384],{"class":80},"    for",[67,1386,1033],{"class":84},[67,1388,112],{"class":80},[67,1390,1391],{"class":84}," stream:\n",[67,1393,1394,1397,1399,1401,1403],{"class":69,"line":204},[67,1395,1396],{"class":84},"        handle(line.rstrip(",[67,1398,637],{"class":145},[67,1400,640],{"class":115},[67,1402,637],{"class":145},[67,1404,1405],{"class":84},"))\n",[67,1407,1408],{"class":69,"line":312},[67,1409,100],{"emptyLinePlaceholder":99},[67,1411,1412],{"class":69,"line":325},[67,1413,100],{"emptyLinePlaceholder":99},[67,1415,1416,1418,1421,1424,1426,1429,1431,1433,1435],{"class":69,"line":337},[67,1417,315],{"class":80},[67,1419,1420],{"class":318}," stream_two",[67,1422,1423],{"class":84},"(argv: list[",[67,1425,331],{"class":115},[67,1427,1428],{"class":84},"], on_out: Callable[[",[67,1430,331],{"class":115},[67,1432,354],{"class":84},[67,1434,357],{"class":115},[67,1436,334],{"class":84},[67,1438,1439,1442,1444,1446,1448,1450,1452],{"class":69,"line":346},[67,1440,1441],{"class":84},"               on_err: Callable[[",[67,1443,331],{"class":115},[67,1445,354],{"class":84},[67,1447,357],{"class":115},[67,1449,1375],{"class":84},[67,1451,368],{"class":115},[67,1453,178],{"class":84},[67,1455,1456,1458,1461,1464,1466,1468,1470,1472,1475,1477,1479,1481],{"class":69,"line":362},[67,1457,454],{"class":80},[67,1459,1460],{"class":84}," subprocess.Popen(argv, ",[67,1462,1463],{"class":194},"stdout",[67,1465,198],{"class":80},[67,1467,474],{"class":84},[67,1469,477],{"class":115},[67,1471,125],{"class":84},[67,1473,1474],{"class":194},"stderr",[67,1476,198],{"class":80},[67,1478,474],{"class":84},[67,1480,477],{"class":115},[67,1482,343],{"class":84},[67,1484,1485,1488,1490,1492,1494,1496,1499,1501,1503,1505,1508,1510,1512],{"class":69,"line":379},[67,1486,1487],{"class":194},"                          stdin",[67,1489,198],{"class":80},[67,1491,474],{"class":84},[67,1493,511],{"class":115},[67,1495,125],{"class":84},[67,1497,1498],{"class":194},"text",[67,1500,198],{"class":80},[67,1502,524],{"class":115},[67,1504,125],{"class":84},[67,1506,1507],{"class":194},"encoding",[67,1509,198],{"class":80},[67,1511,537],{"class":145},[67,1513,343],{"class":84},[67,1515,1516,1519,1521,1523,1526,1528],{"class":69,"line":406},[67,1517,1518],{"class":194},"                          errors",[67,1520,198],{"class":80},[67,1522,550],{"class":145},[67,1524,1525],{"class":84},") ",[67,1527,588],{"class":80},[67,1529,591],{"class":84},[67,1531,1532,1535,1537],{"class":69,"line":422},[67,1533,1534],{"class":84},"        threads ",[67,1536,198],{"class":80},[67,1538,1539],{"class":84}," [\n",[67,1541,1542,1545,1548,1550,1553,1556,1558,1561,1564,1566,1568],{"class":69,"line":428},[67,1543,1544],{"class":84},"            threading.Thread(",[67,1546,1547],{"class":194},"target",[67,1549,198],{"class":80},[67,1551,1552],{"class":84},"_pump, ",[67,1554,1555],{"class":194},"args",[67,1557,198],{"class":80},[67,1559,1560],{"class":84},"(proc.stdout, on_out), ",[67,1562,1563],{"class":194},"daemon",[67,1565,198],{"class":80},[67,1567,524],{"class":115},[67,1569,1570],{"class":84},"),\n",[67,1572,1573,1575,1577,1579,1581,1583,1585,1588,1590,1592,1594],{"class":69,"line":451},[67,1574,1544],{"class":84},[67,1576,1547],{"class":194},[67,1578,198],{"class":80},[67,1580,1552],{"class":84},[67,1582,1555],{"class":194},[67,1584,198],{"class":80},[67,1586,1587],{"class":84},"(proc.stderr, on_err), ",[67,1589,1563],{"class":194},[67,1591,198],{"class":80},[67,1593,524],{"class":115},[67,1595,1570],{"class":84},[67,1597,1598],{"class":69,"line":460},[67,1599,1600],{"class":84},"        ]\n",[67,1602,1603,1605,1608,1610],{"class":69,"line":466},[67,1604,615],{"class":80},[67,1606,1607],{"class":84}," t ",[67,1609,112],{"class":80},[67,1611,1612],{"class":84}," threads:\n",[67,1614,1615],{"class":69,"line":482},[67,1616,1617],{"class":84},"            t.start()\n",[67,1619,1620,1622,1624,1626],{"class":69,"line":501},[67,1621,615],{"class":80},[67,1623,1607],{"class":84},[67,1625,112],{"class":80},[67,1627,1612],{"class":84},[67,1629,1630],{"class":69,"line":516},[67,1631,1632],{"class":84},"            t.join()\n",[67,1634,1635,1637],{"class":69,"line":529},[67,1636,662],{"class":80},[67,1638,1639],{"class":84}," proc.returncode\n",[10,1641,1642,1643,1646,1647,225,1650,1653,1654,1657,1658,1661,1662,27],{},"The callbacks run on the pumping threads, so keep them simple — appending to a list, putting onto a ",[14,1644,1645],{},"queue.Queue",", or printing. ",[14,1648,1649],{},"print",[14,1651,1652],{},"typer.echo"," are safe to call from threads; building up a shared data structure needs a lock or a queue. If your CLI already uses asyncio, ",[14,1655,1656],{},"asyncio.create_subprocess_exec"," with two ",[14,1659,1660],{},"readline()"," tasks does the same job without threads; see ",[23,1663,1665],{"href":1664},"\u002Fcli-runtime-systems-integration\u002Fconcurrency-and-async-in-python-clis\u002Frunning-async-code-in-typer-and-click\u002F","running async code in Typer and Click",[29,1667,1669],{"id":1668},"ux-considerations","UX considerations",[34,1671,1672,1682,1692,1705,1711,1725],{},[37,1673,1674,1677,1678,1681],{},[1178,1675,1676],{},"Prefix each line with its source."," ",[14,1679,1680],{},"build │ ..."," makes it obvious which lines are yours and which are the child's, especially when a command runs several steps in sequence.",[37,1683,1684,1687,1688,1691],{},[1178,1685,1686],{},"Stream to stderr."," Live progress is narration, not results. Keeping it on stderr lets ",[14,1689,1690],{},"mytool build > report.txt"," still capture only your command's actual output.",[37,1693,1694,1700,1701,1704],{},[1178,1695,1696,1697,27],{},"Filter by default, pass everything with ",[14,1698,1699],{},"--verbose"," Most child output is noise to your user. Show the lines that matter (steps, warnings, errors) and let ",[14,1702,1703],{},"-v"," reveal the rest.",[37,1706,1707,1710],{},[1178,1708,1709],{},"Keep a tail for failures."," When the child fails, reprint its last lines under a clear heading. That is often the only part the user reads.",[37,1712,1713,1716,1717,1720,1721,27],{},[1178,1714,1715],{},"Do not fight a progress display."," If you use a Rich progress bar or status spinner, print child lines through its console (",[14,1718,1719],{},"progress.console.print(...)",") so they appear above the bar instead of tearing it. The details are in ",[23,1722,1724],{"href":1723},"\u002Fadvanced-input-parsing-user-experience\u002Finteractive-terminal-ui-with-rich\u002Fadding-progress-bars-and-spinners-to-python-clis\u002F","adding progress bars and spinners to Python CLIs",[37,1726,1727,1730,1731,1734,1735,27],{},[1178,1728,1729],{},"Strip carriage-return redraws."," Tools that draw their own progress bars emit ",[14,1732,1733],{},"\\r"," to overwrite a line. In a log those become enormous single lines; keep only the text after the last ",[14,1736,1733],{},[29,1738,1740],{"id":1739},"testing-the-behaviour","Testing the behaviour",[10,1742,1743,1744,1747],{},"Two properties are worth pinning: lines arrive ",[1178,1745,1746],{},"before"," the child exits, and the returned tail and exit code are right. The first needs a real child that pauses between lines, so the test can observe the timing:",[58,1749,1751],{"className":60,"code":1750,"language":62,"meta":63,"style":63},"# tests\u002Ftest_stream.py\nimport sys\nimport time\n\nfrom mytool.stream import stream_command\n\nCHILD = (\n    \"import sys, time\\n\"\n    \"for i in range(3):\\n\"\n    \"    print(i, flush=True)\\n\"\n    \"    time.sleep(0.3)\\n\"\n    \"sys.exit(2)\\n\"\n)\n\n\ndef test_lines_arrive_while_child_runs():\n    seen: list[tuple[str, float]] = []\n    start = time.monotonic()\n    code, tail = stream_command(\n        [sys.executable, \"-c\", CHILD],\n        on_line=lambda line: seen.append((line, time.monotonic() - start)),\n    )\n    assert code == 2\n    assert [line for line, _ in seen] == [\"0\", \"1\", \"2\"]\n    # The first line arrived well before the child finished (~0.9s).\n    assert seen[0][1] \u003C 0.6\n\n\ndef test_tail_is_bounded():\n    code, tail = stream_command(\n        [sys.executable, \"-c\", \"for i in range(500): print(i)\"],\n        on_line=lambda _: None,\n        keep_last=5,\n    )\n    assert code == 0\n    assert tail == [\"495\", \"496\", \"497\", \"498\", \"499\"]\n",[14,1752,1753,1758,1764,1770,1774,1784,1788,1798,1808,1817,1826,1835,1844,1848,1852,1856,1866,1886,1896,1905,1919,1935,1940,1952,1989,1994,2017,2021,2025,2034,2042,2055,2068,2080,2084,2095],{"__ignoreMap":63},[67,1754,1755],{"class":69,"line":70},[67,1756,1757],{"class":73},"# tests\u002Ftest_stream.py\n",[67,1759,1760,1762],{"class":69,"line":77},[67,1761,81],{"class":80},[67,1763,85],{"class":84},[67,1765,1766,1768],{"class":69,"line":88},[67,1767,81],{"class":80},[67,1769,93],{"class":84},[67,1771,1772],{"class":69,"line":96},[67,1773,100],{"emptyLinePlaceholder":99},[67,1775,1776,1778,1780,1782],{"class":69,"line":103},[67,1777,251],{"class":80},[67,1779,719],{"class":84},[67,1781,81],{"class":80},[67,1783,724],{"class":84},[67,1785,1786],{"class":69,"line":134},[67,1787,100],{"emptyLinePlaceholder":99},[67,1789,1790,1793,1795],{"class":69,"line":164},[67,1791,1792],{"class":115},"CHILD",[67,1794,371],{"class":80},[67,1796,1797],{"class":84}," (\n",[67,1799,1800,1803,1805],{"class":69,"line":181},[67,1801,1802],{"class":145},"    \"import sys, time",[67,1804,640],{"class":115},[67,1806,1807],{"class":145},"\"\n",[67,1809,1810,1813,1815],{"class":69,"line":204},[67,1811,1812],{"class":145},"    \"for i in range(3):",[67,1814,640],{"class":115},[67,1816,1807],{"class":145},[67,1818,1819,1822,1824],{"class":69,"line":312},[67,1820,1821],{"class":145},"    \"    print(i, flush=True)",[67,1823,640],{"class":115},[67,1825,1807],{"class":145},[67,1827,1828,1831,1833],{"class":69,"line":325},[67,1829,1830],{"class":145},"    \"    time.sleep(0.3)",[67,1832,640],{"class":115},[67,1834,1807],{"class":145},[67,1836,1837,1840,1842],{"class":69,"line":337},[67,1838,1839],{"class":145},"    \"sys.exit(2)",[67,1841,640],{"class":115},[67,1843,1807],{"class":145},[67,1845,1846],{"class":69,"line":346},[67,1847,161],{"class":84},[67,1849,1850],{"class":69,"line":362},[67,1851,100],{"emptyLinePlaceholder":99},[67,1853,1854],{"class":69,"line":379},[67,1855,100],{"emptyLinePlaceholder":99},[67,1857,1858,1860,1863],{"class":69,"line":406},[67,1859,315],{"class":80},[67,1861,1862],{"class":318}," test_lines_arrive_while_child_runs",[67,1864,1865],{"class":84},"():\n",[67,1867,1868,1871,1873,1875,1878,1881,1883],{"class":69,"line":422},[67,1869,1870],{"class":84},"    seen: list[tuple[",[67,1872,331],{"class":115},[67,1874,125],{"class":84},[67,1876,1877],{"class":115},"float",[67,1879,1880],{"class":84},"]] ",[67,1882,198],{"class":80},[67,1884,1885],{"class":84}," []\n",[67,1887,1888,1891,1893],{"class":69,"line":428},[67,1889,1890],{"class":84},"    start ",[67,1892,198],{"class":80},[67,1894,1895],{"class":84}," time.monotonic()\n",[67,1897,1898,1900,1902],{"class":69,"line":451},[67,1899,946],{"class":84},[67,1901,198],{"class":80},[67,1903,1904],{"class":84}," stream_command(\n",[67,1906,1907,1910,1913,1915,1917],{"class":69,"line":460},[67,1908,1909],{"class":84},"        [sys.executable, ",[67,1911,1912],{"class":145},"\"-c\"",[67,1914,125],{"class":84},[67,1916,1792],{"class":115},[67,1918,334],{"class":84},[67,1920,1921,1924,1927,1930,1932],{"class":69,"line":466},[67,1922,1923],{"class":194},"        on_line",[67,1925,1926],{"class":80},"=lambda",[67,1928,1929],{"class":84}," line: seen.append((line, time.monotonic() ",[67,1931,1041],{"class":80},[67,1933,1934],{"class":84}," start)),\n",[67,1936,1937],{"class":69,"line":482},[67,1938,1939],{"class":84},"    )\n",[67,1941,1942,1945,1947,1949],{"class":69,"line":501},[67,1943,1944],{"class":80},"    assert",[67,1946,979],{"class":84},[67,1948,172],{"class":80},[67,1950,1951],{"class":115}," 2\n",[67,1953,1954,1956,1959,1961,1964,1966,1969,1971,1974,1977,1979,1981,1983,1986],{"class":69,"line":516},[67,1955,1944],{"class":80},[67,1957,1958],{"class":84}," [line ",[67,1960,106],{"class":80},[67,1962,1963],{"class":84}," line, _ ",[67,1965,112],{"class":80},[67,1967,1968],{"class":84}," seen] ",[67,1970,172],{"class":80},[67,1972,1973],{"class":84}," [",[67,1975,1976],{"class":145},"\"0\"",[67,1978,125],{"class":84},[67,1980,856],{"class":145},[67,1982,125],{"class":84},[67,1984,1985],{"class":145},"\"2\"",[67,1987,1988],{"class":84},"]\n",[67,1990,1991],{"class":69,"line":529},[67,1992,1993],{"class":73},"    # The first line arrived well before the child finished (~0.9s).\n",[67,1995,1996,1998,2001,2004,2007,2009,2011,2014],{"class":69,"line":542},[67,1997,1944],{"class":80},[67,1999,2000],{"class":84}," seen[",[67,2002,2003],{"class":115},"0",[67,2005,2006],{"class":84},"][",[67,2008,122],{"class":115},[67,2010,391],{"class":84},[67,2012,2013],{"class":80},"\u003C",[67,2015,2016],{"class":115}," 0.6\n",[67,2018,2019],{"class":69,"line":555},[67,2020,100],{"emptyLinePlaceholder":99},[67,2022,2023],{"class":69,"line":571},[67,2024,100],{"emptyLinePlaceholder":99},[67,2026,2027,2029,2032],{"class":69,"line":582},[67,2028,315],{"class":80},[67,2030,2031],{"class":318}," test_tail_is_bounded",[67,2033,1865],{"class":84},[67,2035,2036,2038,2040],{"class":69,"line":594},[67,2037,946],{"class":84},[67,2039,198],{"class":80},[67,2041,1904],{"class":84},[67,2043,2044,2046,2048,2050,2053],{"class":69,"line":612},[67,2045,1909],{"class":84},[67,2047,1912],{"class":145},[67,2049,125],{"class":84},[67,2051,2052],{"class":145},"\"for i in range(500): print(i)\"",[67,2054,334],{"class":84},[67,2056,2057,2059,2061,2064,2066],{"class":69,"line":626},[67,2058,1923],{"class":194},[67,2060,1926],{"class":80},[67,2062,2063],{"class":84}," _: ",[67,2065,357],{"class":115},[67,2067,343],{"class":84},[67,2069,2070,2073,2075,2078],{"class":69,"line":647},[67,2071,2072],{"class":194},"        keep_last",[67,2074,198],{"class":80},[67,2076,2077],{"class":115},"5",[67,2079,343],{"class":84},[67,2081,2082],{"class":69,"line":653},[67,2083,1939],{"class":84},[67,2085,2086,2088,2090,2092],{"class":69,"line":659},[67,2087,1944],{"class":80},[67,2089,979],{"class":84},[67,2091,172],{"class":80},[67,2093,2094],{"class":115}," 0\n",[67,2096,2097,2099,2102,2104,2106,2109,2111,2114,2116,2119,2121,2124,2126,2129],{"class":69,"line":1140},[67,2098,1944],{"class":80},[67,2100,2101],{"class":84}," tail ",[67,2103,172],{"class":80},[67,2105,1973],{"class":84},[67,2107,2108],{"class":145},"\"495\"",[67,2110,125],{"class":84},[67,2112,2113],{"class":145},"\"496\"",[67,2115,125],{"class":84},[67,2117,2118],{"class":145},"\"497\"",[67,2120,125],{"class":84},[67,2122,2123],{"class":145},"\"498\"",[67,2125,125],{"class":84},[67,2127,2128],{"class":145},"\"499\"",[67,2130,1988],{"class":84},[10,2132,2133,2134,2137,2138,27],{},"Timing assertions are inherently a little fragile on loaded CI machines, so leave a generous margin — the point is to catch a regression to \"everything at the end\", which shows up as the first line arriving after roughly a second. For the command layer, patch ",[14,2135,2136],{},"stream_command"," and feed it canned lines, as in ",[23,2139,2141],{"href":2140},"\u002Fmodern-python-cli-frameworks-architecture\u002Ftesting-python-cli-applications\u002Ftesting-click-commands-with-clirunner\u002F","testing Click commands with CliRunner",[29,2143,2145],{"id":2144},"conclusion","Conclusion",[10,2147,2148,2149,2151,2152,2155,2156,27],{},"Live output is three decisions: iterate over a ",[14,2150,20],{}," pipe rather than waiting on ",[14,2153,2154],{},"run()",", make the child flush per line rather than per block, and read two streams concurrently or merge them into one. Get those right and a long-running child feels responsive, your memory use stays flat, and failures come with the context needed to fix them. When the child might run too long, add a deadline using ",[23,2157,2159],{"href":2158},"\u002Fcli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002Fhandling-subprocess-timeouts-and-exit-codes\u002F","handling subprocess timeouts and exit codes",[29,2161,2163],{"id":2162},"frequently-asked-questions","Frequently asked questions",[2165,2166,2168,2169,2172],"h3",{"id":2167},"do-i-still-need-iterprocstdoutreadline","Do I still need ",[14,2170,2171],{},"iter(proc.stdout.readline, \"\")","?",[10,2174,2175,2176,2178,2179,2181],{},"Older answers recommend ",[14,2177,2171],{}," because Python 2's file iterator used a read-ahead buffer. In Python 3 the plain ",[14,2180,106],{}," loop over a text-mode pipe yields each line as soon as it is complete, so the workaround is unnecessary.",[2165,2183,2185],{"id":2184},"how-do-i-show-colour-from-a-child-that-disables-it-when-piped","How do I show colour from a child that disables it when piped?",[10,2187,2188,2189,2192,2193,2196,2197,27],{},"Many tools accept a flag or variable to force colour even when not on a terminal, such as ",[14,2190,2191],{},"--color=always"," or ",[14,2194,2195],{},"FORCE_COLOR=1",". Pass it only when your own stderr is a terminal, so colour codes do not end up in log files. See ",[23,2198,2200],{"href":2199},"\u002Fadvanced-input-parsing-user-experience\u002Fcross-platform-terminal-compatibility\u002Frespecting-no-color-and-force-color\u002F","respecting NO_COLOR and FORCE_COLOR",[2165,2202,2204],{"id":2203},"can-i-stream-output-and-also-enforce-a-timeout","Can I stream output and also enforce a timeout?",[10,2206,2207,2208,2210,2211,2214,2215,2218],{},"Yes, but ",[14,2209,20],{}," has no timeout on iteration. Start a ",[14,2212,2213],{},"threading.Timer"," that calls ",[14,2216,2217],{},"proc.kill()"," when the deadline passes; the pipe then closes and your loop ends. The timeout guide shows a version that kills the whole process group.",[2165,2220,2222],{"id":2221},"should-i-write-the-full-log-to-a-file-as-well","Should I write the full log to a file as well?",[10,2224,2225,2226,2230],{},"For long jobs, yes: stream the filtered view to the terminal and write every line to a log file in your app's state directory, then print its path on failure. The guide on ",[23,2227,2229],{"href":2228},"\u002Fadvanced-input-parsing-user-experience\u002Fstructured-logging-for-cli-apps\u002Fwriting-rotating-log-files-from-a-cli\u002F","writing rotating log files from a CLI"," covers where that file should live.",[29,2232,2234],{"id":2233},"related","Related",[34,2236,2237,2244,2249,2254,2260],{},[37,2238,2239,2240],{},"Up: ",[23,2241,2243],{"href":2242},"\u002Fcli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002F","Running subprocesses from Python CLIs",[37,2245,2246],{},[23,2247,2248],{"href":25},"Calling external commands safely with subprocess",[37,2250,2251],{},[23,2252,2253],{"href":2158},"Handling subprocess timeouts and exit codes",[37,2255,2256],{},[23,2257,2259],{"href":2258},"\u002Fadvanced-input-parsing-user-experience\u002Fworking-with-stdin-stdout-and-pipes\u002Fdetecting-tty-and-adapting-output\u002F","Detecting a TTY and adapting output",[37,2261,2262],{},[23,2263,2264],{"href":1723},"Adding progress bars and spinners to Python CLIs",[2266,2267,2268],"style",{},"html pre.shiki code .sJ8bj, html code.shiki .sJ8bj{--shiki-default:#6A737D;--shiki-dark:#6A737D}html pre.shiki code .szBVR, html code.shiki .szBVR{--shiki-default:#D73A49;--shiki-dark:#F97583}html pre.shiki code .sVt8B, html code.shiki .sVt8B{--shiki-default:#24292E;--shiki-dark:#E1E4E8}html pre.shiki code .sj4cs, html code.shiki .sj4cs{--shiki-default:#005CC5;--shiki-dark:#79B8FF}html pre.shiki code .sZZnC, html code.shiki .sZZnC{--shiki-default:#032F62;--shiki-dark:#9ECBFF}html pre.shiki code .s4XuR, html code.shiki .s4XuR{--shiki-default:#E36209;--shiki-dark:#FFAB70}html .default .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html .dark .shiki span {color: var(--shiki-dark);background: var(--shiki-dark-bg);font-style: var(--shiki-dark-font-style);font-weight: var(--shiki-dark-font-weight);text-decoration: var(--shiki-dark-text-decoration);}html.dark .shiki span {color: var(--shiki-dark);background: var(--shiki-dark-bg);font-style: var(--shiki-dark-font-style);font-weight: var(--shiki-dark-font-weight);text-decoration: var(--shiki-dark-text-decoration);}html pre.shiki code .sScJk, html code.shiki .sScJk{--shiki-default:#6F42C1;--shiki-dark:#B392F0}",{"title":63,"searchDepth":77,"depth":77,"links":2270},[2271,2272,2273,2274,2275,2276,2277,2278,2285],{"id":31,"depth":77,"text":32},{"id":215,"depth":77,"text":216},{"id":1172,"depth":77,"text":1173},{"id":1253,"depth":77,"text":1254},{"id":1668,"depth":77,"text":1669},{"id":1739,"depth":77,"text":1740},{"id":2144,"depth":77,"text":2145},{"id":2162,"depth":77,"text":2163,"children":2279},[2280,2282,2283,2284],{"id":2167,"depth":88,"text":2281},"Do I still need iter(proc.stdout.readline, \"\")?",{"id":2184,"depth":88,"text":2185},{"id":2203,"depth":88,"text":2204},{"id":2221,"depth":88,"text":2222},{"id":2233,"depth":77,"text":2234},"2026-09-18","Show a child process’s output live from a Python CLI: Popen line iteration, beating pipe buffering, prefixing lines, reading two streams without deadlock.","intermediate",false,"md",{},"\u002Fcli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002Fstreaming-subprocess-output-in-real-time",{"title":5,"description":2287},"cli-runtime-systems-integration\u002Frunning-subprocesses-from-python-clis\u002Fstreaming-subprocess-output-in-real-time\u002Findex",[2296,2297,2298,2299],"subprocess","streaming","output","cli","2d0U3V9bu62qXFtAVPpwCJWPu8PIL_SghPetdgGpu0I",[2302,2305,2308,2311,2314,2317,2320,2323,2326,2329,2332,2335,2338,2341,2344,2347,2350,2353,2356,2359,2362,2365,2368,2371,2374,2377,2380,2383,2386,2389,2392,2395,2398,2401,2404,2407,2410,2413,2416,2419,2422,2425,2428,2431,2434,2437,2440,2443,2446,2449,2452,2455,2458,2461,2464,2467,2470,2473,2476,2479,2482,2485,2488,2491,2494,2497,2500,2503,2506,2509,2512,2515,2518,2521,2524,2527,2530,2533,2536,2539,2542,2545,2548,2551,2554,2557,2560,2563,2566,2569,2572,2575,2578,2581,2584,2587,2590,2593,2596,2599,2602,2605,2608,2611,2614,2617,2620,2623,2626,2629,2632,2635,2638,2641,2644,2647,2650,2653,2656,2659,2662,2665,2668,2671,2674,2677,2678,2681,2684,2687,2690,2693,2696,2699,2702,2705,2708,2711,2714,2717,2720,2723,2726,2729,2732,2735,2738,2741,2744,2747,2750,2753,2756,2759,2762,2765,2768,2771,2774,2777,2780,2783,2786,2789,2792,2795,2798,2801,2804,2807,2810,2813,2816,2819,2822,2825,2828,2831,2834,2837,2840,2843,2846,2849,2852,2855,2858,2861,2864,2867,2870,2873,2876,2879,2882,2885,2888,2891,2894,2897,2900,2903,2906,2909,2912,2915,2918,2921,2924,2927,2930,2933,2936,2939,2942,2945,2948,2951,2954,2957,2960,2963,2966,2969,2972,2975,2978,2981,2984,2987,2990,2993,2996,2999,3002,3005,3008,3011,3014,3017,3020,3023,3026,3029,3032,3035,3038,3041,3044,3047,3050,3053,3056,3059,3062,3065,3068,3071,3074,3077,3080,3083,3086,3089,3092,3095,3098,3101,3104,3107,3110,3113,3116,3119,3122,3125,3128,3131,3134,3137,3140,3143],{"path":2303,"title":2304},"\u002Fabout","About Python CLI Toolcraft",{"path":2306,"title":2307},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies","Advanced Argument Validation Strategies",{"path":2309,"title":2310},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies\u002Fparsing-nested-json-arguments-in-python-clis","Parsing Nested JSON Args in Python CLIs",{"path":2312,"title":2313},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies\u002Fvalidating-dates-and-durations-in-cli-arguments","Validating Dates and Durations in Python CLI Arguments",{"path":2315,"title":2316},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies\u002Fvalidating-dependent-and-conflicting-options","Validating Dependent and Conflicting CLI Options",{"path":2318,"title":2319},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies\u002Fvalidating-file-and-directory-paths-in-clis","Validating File and Directory Paths in CLIs",{"path":2321,"title":2322},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies\u002Fvalidating-urls-hosts-and-ports","Validating URLs, Hosts and Ports in Python CLI Arguments",{"path":2324,"title":2325},"\u002Fadvanced-input-parsing-user-experience\u002Fadvanced-argument-validation-strategies\u002Fwriting-custom-click-parameter-types","Writing Custom Click Parameter Types",{"path":2327,"title":2328},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual\u002Fbrowsing-records-with-a-textual-datatable","Browsing Records with a Textual DataTable Picker",{"path":2330,"title":2331},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual\u002Fbuilding-your-first-textual-app","Building Your First Textual App for a Python CLI",{"path":2333,"title":2334},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual\u002Fchoosing-between-a-cli-a-prompt-flow-and-a-tui","Choosing Between a CLI, a Prompt Flow and a TUI",{"path":2336,"title":2337},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual","Building Terminal UIs with Textual for Python CLIs",{"path":2339,"title":2340},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual\u002Frunning-background-work-in-textual-with-workers","Running Background Work in Textual with Workers",{"path":2342,"title":2343},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual\u002Fstyling-textual-apps-with-tcss","Styling Textual Apps with TCSS",{"path":2345,"title":2346},"\u002Fadvanced-input-parsing-user-experience\u002Fbuilding-terminal-uis-with-textual\u002Ftesting-textual-apps-with-pilot","Testing Textual Apps with Pilot",{"path":2348,"title":2349},"\u002Fadvanced-input-parsing-user-experience\u002Fcli-help-output-and-documentation\u002Fadding-examples-and-epilogs-to-help-output","Adding Examples and Epilogs to Help Output",{"path":2351,"title":2352},"\u002Fadvanced-input-parsing-user-experience\u002Fcli-help-output-and-documentation\u002Fdocumenting-environment-variables-in-help","Documenting Environment Variables in CLI Help",{"path":2354,"title":2355},"\u002Fadvanced-input-parsing-user-experience\u002Fcli-help-output-and-documentation\u002Fgenerating-man-pages-and-docs-from-a-cli","Generating Man Pages and Docs from a CLI",{"path":2357,"title":2358},"\u002Fadvanced-input-parsing-user-experience\u002Fcli-help-output-and-documentation","CLI Help Output and 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