4.9: Parameters and Arguments
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Some of the built-in functions we have seen require arguments. For example, when you call
math.sin you pass a number as an argument. Some functions take more than one argument:
math.pow takes two, the base and the exponent.
Inside the function, the arguments are assigned to variables called parameters. Here is an example of a user-defined function that takes an argument:
def print_twice(bruce): print(bruce) print(bruce)
This function assigns the argument to a parameter named
bruce. When the function is called, it prints the value of the parameter (whatever it is) twice.
This function works with any value that can be printed.
>>> print_twice('Spam') Spam Spam >>> print_twice(17) 17 17 >>> import math >>> print_twice(math.pi) 3.141592653589793 3.141592653589793
The same rules of composition that apply to built-in functions also apply to user-defined functions, so we can use any kind of expression as an argument for
>>> print_twice('Spam '*4) Spam Spam Spam Spam Spam Spam Spam Spam >>> print_twice(math.cos(math.pi)) -1.0 -1.0
The argument is evaluated before the function is called, so in the examples the expressions
Spam '*4 and
math.cos(math.pi) are only evaluated once.
You can also use a variable as an argument:
>>> michael = 'Eric, the half a bee.' >>> print_twice(michael) Eric, the half a bee. Eric, the half a bee.
The name of the variable we pass as an argument (
michael) has nothing to do with the name of the parameter (
bruce). It doesn't matter what the value was called back home (in the caller); here in
print_twice, we call everybody