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pythonpython-typing

How do I add default parameters to functions when using type hinting?

If I have a function like this:

def foo(name, opts={}):
  pass

And I want to add type hints to the parameters, how do I do it? The way I assumed gives me a syntax error:

def foo(name: str, opts={}: dict) -> str:
  pass

The following doesn't throw a syntax error but it doesn't seem like the intuitive way to handle this case:

def foo(name: str, opts: dict={}) -> str:
  pass

I can't find anything in the typing documentation or on a Google search.

Edit: I didn't know how default arguments worked in Python, but for the sake of this question, I will keep the examples above. In general it's much better to do the following:

def foo(name: str, opts: dict=None) -> str:
  if not opts:
    opts={}
  pass
Solution

  • Your second way is correct.

    def foo(opts: dict = {}):
    pass

print(foo.__annotations__)

    this outputs

    {'opts': <class 'dict'>}

    Although it is not explicitly mentioned in PEP 484, type hints are a specific use of function annotations, as outlined in PEP 3107. The syntax section clearly demonstrates that keyword arguments can be annotated in this manner.

    I strongly advise against using mutable keyword arguments. More information here.