How to know function return type and argument types?

Question

Well things have changed a little bit since 2011! Now there's type hints in Python 3.5 which you can use to annotate arguments and return the type of your function. For example this:

def greeting(name):
  return 'Hello, {}'.format(name)

can now be written as this:

def greeting(name: str) -> str:
  return 'Hello, {}'.format(name)

As you can now see types, there's some sort of optional static type checking which will help you and your type checker to investigate your code.

for more explanation I suggest to take a look at the blog post on type hints in PyCharm blog.

This is how dynamic languages work. It is not always a good thing though, especially if the documentation is poor - anyone tried to use a poorly documented python framework? Sometimes you have to revert to reading the source.

Here are some strategies to avoid problems with duck typing:

• create a language for your problem domain
• this will help you to name stuff properly
• use types to represent concepts in your domain language
• name function parameters using the domain language vocabulary

Also, one of the most important points:

• keep data as local as possible!

There should only be a few well-defined and documented types being passed around. Anything else should be obvious by looking at the code: Don't have weird parameter types coming from far away that you can't figure out by looking in the vicinity of the code...

Related, (and also related to docstrings), there is a technique in python called doctests. Use that to document how your methods are expected to be used - and have nice unit test coverage at the same time!

I attended a coursera course, there was lesson in which, we were taught about design recipe.

Below docstring format I found preety useful.

def area(base, height):
    '''(number, number ) -> number    #**TypeContract**
    Return the area of a tring with dimensions base   #**Description**
    and height

    >>>area(10,5)          #**Example **
    25.0
    >>area(2.5,3)
    3.75
    '''
    return (base * height) /2 

I think if docstrings are written in this way, it might help a lot to developers.

Link to video [Do watch the video] : https://www.youtube.com/watch?v=QAPg6Vb_LgI

Yes, you should use docstrings to make your classes and functions more friendly to other programmers:

More: http://www.python.org/dev/peps/pep-0257/#what-is-a-docstring

Some editors allow you to see docstrings while typing, so it really makes work easier.