11.5.1. Positional Arguments

These are the standard vanilla parameters that we are all familiar with. Positional arguments must be passed in the exact order that they are defined for the functions that are called. Also, without the presence of any default arguments (see next section), the exact number of arguments passed to a function (call) must be exactly the number declared:

>>> def foo(who):       # defined for only 1 argument
…       print 'Hello', who
…
>>> foo()               # 0 arguments… BAD
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: not enough arguments; expected 1, got 0
>>>
>>> foo('World!')       # 1 argument… WORKS
Hello World!
>>>
>>> foo('Mr.', 'World!')# 2 arguments… BAD
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: too many arguments; expected 1, got 2

The foo() function has one positional argument. That means that any call to foo() must have exactly one argument, no more, no less. You will become extremely familiar with TypeError otherwise. Note how informative the Python errors are. As a general rule, all positional arguments for a function must be provided whenever you call it. They may be passed into the function call in position or out-of-position, granted that a keyword argument is provided to match it to its proper position in the argument list (review Section 11.2.2). Default arguments, however, do not have to be provided because of their nature.