Python之function

1　　Function

　　a function is a device that groups a set of statements so they can be run more than once in a program.

　　　

　　1)　　def statements　　　

def <name>(arg1, arg2,... argN):
    <statements>

　　Function bodies often contain a return statement:　　

def <name>(arg1, arg2,... argN):
    ...
    return <value>

　　2)　　def executes at runtime　　

if test:
    def func():            # Define func this way
        ...
else:
    def func():            # Or else this way
        ...
...
func()                     # Call the version selected and buil

　　One way to understand this code is to realize that the def is much like an = statement: it simply assigns a name at runtime.

　　Unlike C, Python functions do not need to be fully defined before the program runs. More generally, defs are not evaluated until they are reached and run, and the code inside defs is not evaluated until the functions are later called.

　　Because function definition happens at runtime, there’s nothing special about function name. What’s important is the object to which it refers:

othername = func 　　# Assign function object
othername()     　　# Call func again

2　　Example 1 - Definitions and Calls

　　1)  definition　　

>>> def times(x,y):
...     return x * y
... 

　　2)  call

>>> times(4,5)　　# Arguments in parentheses
20

>>> x = times(3.14,4)
>>> x            # Save the result object
12.56
    

>>> times('Ha',4)　　# Functions are "typeless"
'HaHaHaHa'

3　　Example 2 - Intersecting Sequences

　　1)  definition　　

>>> def intersect(seq1,seq2):
...     res = []        　　　　 # Start empty
...     for x in seq1:　　　　　 # Scan seq1
...       if x in seq2:　　　　　# Common item?
...         res.append(x)　　　　# Add to end
...     return res
... 

　　2)  call　　

>>> s1 = "SPAM"
>>> s2 = "SCAM"
>>> intersect(s1,s2)     # Strings
['S', 'A', 'M']

>>> [x for x in s1 if x in s2]
['S', 'A', 'M']

　　3)  polymorphism revisited

>>> x = intersect([1,2,3],(1,4))　　# Mixed types
>>> x　　　　　　　　　　　　　　　　　　# Saved result object
[1]