关于Python3中__pycache__文件夹生成异常的技术咨询

Great question! Let’s break down exactly how Python handles bytecode caching and why you’re seeing this inconsistent behavior with your scripts.

First: When Does Python Generate .pyc Files? #

The key point here is: Python only generates .pyc files for modules that are imported into another script, not for the script you run directly as the main program.

Here’s what likely happened in your first run:

• You ran one script (let’s call it script_a.py) that imported the other script (script_b.py). Since script_b.py was loaded as a module, Python compiled it to bytecode and stored the .pyc file in __pycache__ to speed up future imports.
• The script you ran directly (script_a.py) is treated as the __main__ module, so Python doesn’t save its bytecode to __pycache__—it assumes you’re executing it directly rather than reusing it as a module elsewhere.

Why Did __pycache__ Disappear After You Deleted It? #

When you deleted the __pycache__ folder and ran the scripts again, you probably weren’t triggering the same import behavior. For example:

• Maybe you ran each script individually (e.g., ./script_a.py then ./script_b.py) instead of running one that imports the other. Since both were treated as __main__ modules, neither generated a .pyc file.
• Even if you ran python3 my_scripts, if that command didn’t involve importing one script into the other, there’s no need for Python to create bytecode caches.

Additional Details to Clarify #

• The #!/usr/bin/python3 shebang line doesn’t affect bytecode generation at all—it just tells your shell which interpreter to use. The same rules apply whether you run the script with ./script_name or python3 script_name.
• Python checks the modification time of your .py file against existing .pyc files. If the .py file hasn’t changed since the last compile, it skips regenerating the .pyc—but since you deleted the cache entirely, this wasn’t the issue here.
• You can force Python to generate a .pyc file for any script using the py_compile module:

  python3 -m py_compile script_name.py
  

• If you’d ever want to disable bytecode generation entirely, you can set the PYTHONDONTWRITEBYTECODE environment variable, but you mentioned you didn’t use this, so it’s not relevant here.

Quick Recap of Your Scenario #

1. First run: One script imported the other → the imported script (as a module) got a .pyc in __pycache__.
2. After deleting the cache: You ran scripts without triggering an import → no modules were loaded, so no __pycache__ or .pyc files were created.

内容的提问来源于stack exchange，提问作者mikeg