Reblog: “Top 10 Mistakes that Python Programmers Make”
Martin Chikilian from Toptal rounds up some common mistakes that Python programmers make.
I have made mistake #1 on multiple occasions:
Common Mistake #1: Misusing expressions as defaults for function arguments
Python allows you to specify that a function argument is optional by providing a default value for it. While this is a great feature of the language, it can lead to some confusion when the default value is mutable. For example, consider this Python function definition:
>>> def foo(bar=): # bar is optional and defaults to  if not specified ... bar.append("baz") # but this line could be problematic, as we'll see... ... return bar
A common mistake is to think that the optional argument will be set to the specified default expression each time the function is called without supplying a value for the optional argument. In the above code, for example, one might expect that calling foo() repeatedly (i.e., without specifying a bar argument) would always return ‘baz’, since the assumption would be that each time foo() is called (without a bar argument specified) bar is set to  (i.e., a new empty list).
I don’t remember for sure, but I’ve probably done something like #5, modifying a list while iterating through it.
If you write Python code, the rest of the article is worth a read