Adding default parameter value with type hint in Python

Question

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  - gt  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     - gt  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  - gt  str    if not opts      opts      pass

User · Accepted Answer

Your second way is correct    def foo opts  dict            pass  print foo   annotations      this outputs    opts    lt class  dict  gt     It s true that s it s not listed in PEP 484  but type hints are an application of function annotations  which are documented in PEP 3107  The syntax section makes it clear that keyword arguments works with function annotations in this way    I strongly advise against using mutable keyword arguments  More information here

User · Answer

I recently saw this one-liner   def foo name  str  opts  dict None  - gt  str      opts      if not opts else opts     pass

User · Answer

If you re using typing  introduced in Python 3 5  you can use typing Optional  where Optional X  is equivalent to Union X  None   It is used to signal that the explicit value of None is allowed   From typing Optional   def foo arg  Optional int    None  - gt  None

[python] Adding default parameter value with type hint in Python

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