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Functions Without returns

In Python functions, return (and yield) statements are optional. When a function doesn’t return a value explicitly, the function exits when control falls off the end of the function body. Technically, all functions return a value; if you don’t provide a return statement, your function returns the None object automatically:

>>> def proc(x):
...     print(x)                 # No return is a None return
...
>>> x = proc('testing 123...')
testing 123...
>>> print(x)
None

Functions such as this without a return are Python’s equivalent of what are called “procedures” in some languages. They’re usually invoked as statements, and the None results are ignored, as they do their business without computing a useful result.

This is worth knowing, because Python won’t tell you if you try to use the result of a function that doesn’t return one. For instance, assigning the result of a list append method won’t raise an error, but you’ll get back None, not the modified list:

>>> list = [1, 2, 3]
>>> list = list.append(4)        # append is a "procedure"
>>> print(list)                  # append changes list in-place
None

As mentioned in Common Coding Gotchas in Chapter 15, such functions do their business as a side effect and are usually designed to be run as statements, not expressions.