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pythonpep8default-arguments

Setting defaults for empty arguments (Python)

Let's say we have the function f and I need the argument b to default to an empty list, but can't set b=[] because of the issue around mutable default args.

Which of these is the most Pythonic, or is there a better way?

def f(a, b=None):
   if not b:
     b = []
   pass

def f(a, b=None):
   b = b or []
   pass
Solution

  • The first form as it reads easier. Without any specific context, you should explicitly test for the default value, to avoid potential truthiness issues with the passed in value.

    def f(a, b=None):
   if b is None:
     b = []
   pass

    From PEP 8, Programming Recommendations:

    Also, beware of writing if x when you really mean if x is not None -- e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might have a type (such as a container) that could be false in a boolean context!

    You can see examples of this approach throughout the cpython repository: