Python BASICS (7) -- Functions

This article describes how to organize statements into functions, the concept of parameters, and their usage in programs.

URL: http://www.cnblogs.com/archimedes/p/python-abstract.html.

Create a function

The built-in callable function can be used to determine whether the function is callable:

>>> import math>>> x=1>>> y=math.sqrt>>> callable(x)False>>> callable(y)True

Use del statements to define functions:

>>> def hello(name):  　　  return 'Hello, '+name+'!'>>> print hello('world')Hello, world!>>> print hello('Gumby')Hello, Gumby!

Compile a function of the fibnacci series:

>>> def fibs(num):   　　 result=[0,1]    　　for i in range(num-2):       　　 result.append(result[-2]+result[-1])   　　 return result>>> fibs(10)[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]>>> fibs(15)[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377]

Assigning values to parameters in a function does not change the values of any external variables:

>>> def try_to_change(n):  　　  n='Mr.Gumby'>>> name='Mrs.Entity'>>> try_to_change(name)>>> name'Mrs.Entity'

Because strings (as well as tuples and numbers) cannot be changed, the parameters will not change. However, what will happen when a variable data structure such as a list is used as a parameter:

>>> name='Mrs.Entity'>>> try_to_change(name)>>> name'Mrs.Entity'>>> def change(n):   　　 n[0]='Mr.Gumby'>>> name=['Mrs.Entity','Mrs.Thing']>>> change(name)>>> name['Mr.Gumby', 'Mrs.Thing']

The parameter has changed, which is an important difference from the previous example.

Do not use the following function again:

>>> Name = ['Mrs. entity ', 'Mrs. thing ']> n = name # Again, simulate parameter passing behavior> n [0] = 'Mr. gumby' # change the list> name ['Mr. gumby', 'Mrs. thing ']

When two variables reference a list at the same time, they do reference a list at the same time. To avoid this situation, you can copy a copy of the list. When slicing is performed in the sequence, the returned slice is always a copy, so if you copy the slice of the entire list, you will get a copy:

>>> names=['Mrs.Entity','Mrs.Thing']>>> n=names[:]>>> n is namesFalse>>> n==namesTrue

Changing n does not affect names:

>>> n[0]='Mr.Gumby'>>> n['Mr.Gumby', 'Mrs.Thing']>>> names['Mrs.Entity', 'Mrs.Thing']>>> change(names[:])>>> names['Mrs.Entity', 'Mrs.Thing']

Keyword parameters and default values

Parameters can be ordered by providing the parameter name (but the parameter name and value must correspond ):

>>> def hello(greeting, name):        print '%s,%s!'%(greeting, name)>>> hello(greeting='hello',name='world!')hello,world!!

The most powerful keyword parameter is that the default value can be provided to the parameter:

>>> def hello_1(greeting='hello',name='world!'):    　　print '%s,%s!'%(greeting,name)>>> hello_1()hello,world!!>>> hello_1('Greetings')Greetings,world!!>>> hello_1('Greeting','universe')Greeting,universe!

If you want greeting to use the default value:

>>> hello_1(name='Gumby')hello,Gumby!

It is not difficult to implement the function by providing any number of parameters:

>>> def print_params(*params):   　　 print params>>> print_params('Testing')('Testing',)>>> print_params(1,2,3)(1, 2, 3)

Mixed common parameters:

>>> def print_params_2(title,*params):   　　 print title   　　 print params>>> print_params_2('params:',1,2,3)params:(1, 2, 3)>>> print_params_2('Nothing:')Nothing:()

The asterisk indicates "collecting other location parameters". If no element is provided for collection, params is an empty tuples.

However, keyword parameters cannot be processed:

>>> print_params_2('Hmm...',something=42)Traceback (most recent call last):  File "<pyshell#112>", line 1, in <module>    print_params_2('Hmm...',something=42)TypeError: print_params_2() got an unexpected keyword argument 'something'

Try "**":

>>> Def print_params (** params): print params >>> print_params (x = 1, y = 2, z = 3) {'y': 2, 'X': 1, 'z': 3 }>>> def parames (x, y, z = 3, * pospar, ** keypar): print x, y, z print pospar print keypar >>> parames (1, 2, 3, 5, 6, 7, foo = 1, bar = 2) 1 2 3 (5, 6, 7) {'foo ': 1, 'bar': 2 }>>> parames (1, 2) 1 2 3 () {}>>> def print_params_3 (** params ): print params >>> print_params_3 (x = 1, y = 2, z = 3) {'y': 2, 'x': 1, 'z ': 3 }>>> # The returned value is a dictionary instead of a tuples >>## composite '#' and '##' >>> def print_params_4 (x, y, z = 3, * pospar, ** keypar): print x, y, z print pospar print keypar >>> print_params_4 (1, 2, 3, 5, 6, 7, foo = 1, bar = 2) 1 2 3 (5, 6, 7) {'foo': 1, 'bar': 2 }>>> print_params_4 (1, 2) 1 2 3 (){}