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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"subprocess.Popen.communicate does not send input","articleBody":"# Bug report\n\n### Bug description:\n\nI will start with saying that documentation for `subprocess.Popen` is not clear. I was using `subprocess.run(..., input=..., ...)` and needed to monitor other things while the created process is running, so I changed it to `subprocess.Popen`. The only way to send input was `Popen.communicate(input=...)`, however I wanted to have a timeout. My initial idea was `.communicate(input=..., timeout=0)`, catch the exception, and then continue with `.wait(timeout=STEP)`, however it was hanging. I worked it around by launching `.communicate` without timeout in a separate thread.\n\nThen, I tested a few platforms and looked at the implementation. On Windows, my initial idea works. On Linux, it hangs. After looking into the implementation, I deduced I shouldn't use `.wait()`, but rather `.communicate()` and set the input only for the first invocation. If that's the intention, it would be great to have it documented.\n\nAnyway, the following example hangs on a few Linux versions and few Python versions, including current main branch:\n\n```python\nimport subprocess\nproc = subprocess.Popen(\n    [\"cat\", \"-\"],\n    stdin=subprocess.PIPE\n)\ntry:\n    proc.communicate(input=\"abcd\\n\".encode(), timeout=0)\nexcept subprocess.TimeoutExpired:\n    pass\nprint(\"it will hang:\")\nproc.communicate()  # initially: proc.wait()\nprint(\"finished\")\n```\n\nIt looks like a bug, and this line: https://github.com/python/cpython/blob/9cd5427d9619b96db20d0347a136b3d331af71ae/Lib/subprocess.py#L2108\nwhich should be changed to:\n```python\n                if self.stdin and not self.stdin.closed and self._input:\n```\nAfter this change, the example above no longer hangs. I'll open PR with the mentioned change.\n\n### CPython versions tested on:\n\n3.12, 3.14, CPython main branch\n\n### Operating systems tested on:\n\nLinux, Windows\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-141477\n* gh-142059\n* gh-142060\n* gh-142101\n* gh-142124\n* gh-142125\n* gh-149003\n* gh-149004\n* gh-149005\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/mrowqa","@type":"Person","name":"mrowqa"},"datePublished":"2025-11-12T18:08:28.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/141473/cpython/issues/141473"}