什么时候刷新.pyc 文件？

我明白这一点。”。“ pyc”文件是纯文本的编译版本。Py”文件，在运行时创建，以使程序运行得更快。然而，我发现了一些事情:

  1. 修改“ py”文件后，程序行为发生变化。这表明“ py”文件已经被编译，或者至少经过某种散列过程，或者比较时间戳，以判断是否应该重新编译它们。
  2. 删除所有”。Pyc”文件(rm *.pyc)有时程序行为会发生变化。这将表明它们没有在“更新”时被编译。派斯。

问题:

  • 它们如何决定何时编译？
  • 是否有办法确保他们在开发过程中有更严格的检查？
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最佳答案

The .pyc files are created (and possibly overwritten) only when that python file is imported by some other script. If the import is called, Python checks to see if the .pyc file's internal timestamp is not older than the corresponding .py file. If it is, it loads the .pyc; if it isn't or if the .pyc does not yet exist, Python compiles the .py file into a .pyc and loads it.

What do you mean by "stricter checking"?

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.pyc files generated whenever the corresponding code elements are imported, and updated if the corresponding code files have been updated. If the .pyc files are deleted, they will be automatically regenerated. However, they are not automatically deleted when the corresponding code files are deleted.

This can cause some really fun bugs during file-level refactors.

First of all, you can end up pushing code that only works on your machine and on no one else's. If you have dangling references to files you deleted, these will still work locally if you don't manually delete the relevant .pyc files because .pyc files can be used in imports. This is compounded with the fact that a properly configured version control system will only push .py files to the central repository, not .pyc files, meaning that your code can pass the "import test" (does everything import okay) just fine and not work on anyone else's computer.

Second, you can have some pretty terrible bugs if you turn packages into modules. When you convert a package (a folder with an __init__.py file) into a module (a .py file), the .pyc files that once represented that package remain. In particular, the __init__.pyc remains. So, if you have the package foo with some code that doesn't matter, then later delete that package and create a file foo.py with some function def bar(): pass and run:

from foo import bar

you get:

ImportError: cannot import name bar

because python is still using the old .pyc files from the foo package, none of which define bar. This can be especially problematic on a web server, where totally functioning code can break because of .pyc files.

As a result of both of these reasons (and possibly others), your deployment code and testing code should delete .pyc files, such as with the following line of bash:

find . -name '*.pyc' -delete

Also, as of python 2.6, you can run python with the -B flag to not use .pyc files. See How to avoid .pyc files? for more details.

See also: How do I remove all .pyc files from a project?


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