Python 2.x gotchas and landmines [closed]

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Expressions in default arguments are calculated when the function is defined, not when it’s called.

Example: consider defaulting an argument to the current time:

>>>import time
>>> def report(when=time.time()):
...     print when
...
>>> report()
1210294387.19
>>> time.sleep(5)
>>> report()
1210294387.19

The when argument doesn’t change. It is evaluated when you define the function. It won’t change until the application is re-started.

Strategy: you won’t trip over this if you default arguments to None and then do something useful when you see it:

>>> def report(when=None):
...     if when is None:
...         when = time.time()
...     print when
...
>>> report()
1210294762.29
>>> time.sleep(5)
>>> report()
1210294772.23

Exercise: to make sure you’ve understood: why is this happening?

>>> def spam(eggs=[]):
...     eggs.append("spam")
...     return eggs
...
>>> spam()
['spam']
>>> spam()
['spam', 'spam']
>>> spam()
['spam', 'spam', 'spam']
>>> spam()
['spam', 'spam', 'spam', 'spam']

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