{"id":1479,"date":"2011-01-31T21:17:11","date_gmt":"2011-02-01T05:17:11","guid":{"rendered":"http:\/\/blog.mozilla.org\/webdev\/?p=1479"},"modified":"2011-02-12T22:02:24","modified_gmt":"2011-02-13T06:02:24","slug":"python-scoping-understanding-legb","status":"publish","type":"post","link":"https:\/\/blog.mozilla.org\/webdev\/2011\/01\/31\/python-scoping-understanding-legb\/","title":{"rendered":"Python scoping: understanding LEGB"},"content":{"rendered":"

Note: this is a repost from my blog. You can find the original post here.<\/a><\/em>
\nUpdate:<\/strong> Thanks to Avazu for a clean suggestion<\/a> and Fred for pointing out I should have indicated a good way to get around this.<\/p>\n

Summary<\/h2>\n

Python scoping fun! Read about LEGB<\/a> to understand the basics of python scoping.<\/p>\n

Beef<\/h2>\n

I never bothered to read about how python scoping works until I hit this. It\u2019s not exactly something to research until you have issues with it. \ud83d\ude42<\/p>\n

I had something like this going on:
\n<\/p>\n

\r\ndef func1(param=None):\r\n    def func2():\r\n        if not param:\r\n            param = 'default'\r\n        print param\r\n    # Just return func2.\r\n    return func2\r\n\r\n\r\nif __name__ == '__main__':\r\n   func1('test')()\r\n<\/pre>\n
<\/code>
\nNote:<\/strong> Actual code was not as straightforward, func2<\/code> was actually a decorator. Admittedly, using the same parameter name is not a must, but it\u2019s still a curiosity. I just wanted to fall back to a default value on run-time.<\/p>\n
If you try to run this in python, here\u2019s what you get:
\n<\/p>\n
\r\n~ $ python test.py \r\nTraceback (most recent call last):\r\n  File \"test.py\", line 11, in \r\n    func1('test')()\r\n  File \"test.py\", line 3, in func2\r\n    if not param:\r\nUnboundLocalError: local variable 'param' referenced before assignment\r\n<\/pre>\n
<\/code><\/p>\n
If you\u2019re curious, you can read about the principles of LEGB.<\/a> You have to understand a bit about compilers and the AST<\/a> to get what\u2019s going on behind the scenes. You might think that replacing lines 3-4 with:
\n<\/p>\n
\r\nparam = param or 'default'\r\n<\/pre>\n
<\/code><\/p>\n
Might work. But no. You can\u2019t assign the same parameter at the local level if the enclosing level defines it. Even this fails:
\n<\/p>\n
\r\nparam = param\r\n<\/pre>\n
<\/code><\/p>\n
Fun, no?<\/p>\n
What to do?<\/h2>\n
There are a few ways to get around this.<\/p>\n