def f(a, L=None):    if L is None:        L = []    L.append(a)    return L

4.7.2. Keyword Arguments

Python里面定义函数的时候,经常会看见诸如 def foo(request, *args, **kwargs)样子的函数

这里需要说明的是*args是一个list,而**kwargs则是一个dict

简单的一个栗子说明一下

a, b, c, d = 1, 2, 3, 4

e, f, g = 5, 6, 7

def f(*args, **kwargs):

    print args, type(args)

    print kwargs, type(kwargs)

f(a, b, c, d, e=e, f= f, g=g, h=a)

#output

[1, 2, 3, 4] list

{‘e‘: 5, ‘f‘: 6, ‘g‘: 7, ‘h‘: 1} dict

4.7.5. Lambda Expressions

Small anonymous functions can be created with the lambda keyword. This function returns the sum of its two arguments: lambda a, b: a+b. Lambda functions can be used wherever function objects are required. They are syntactically restricted to a single expression. Semantically, they are just syntactic sugar for a normal function definition. Like nested function definitions, lambda functions can reference variables from the containing scope:

当一些函数简单到不需要专门的去定义一个函数的时候,可以用lambda临时的来一发,比如说这样

>>> pairs = [(1, ‘one‘), (2, ‘two‘), (3, ‘three‘), (4, ‘four‘)]>>> pairs.sort(key=lambda pair: pair[1])>>> pairs[(4, ‘four‘), (1, ‘one‘), (3, ‘three‘), (2, ‘two‘)]
又比如:
f = lambda x, y: x + y # 冒号左边的是参数,右边的返回的值

f(1, 2) # 将会得到1和2的和3

4.7.6. Documentation Strings

一份好的代码,往往不需要注释都清晰明了一目了然,但当项目代码变得复杂,高度的模块化了的时候,嵌套引用有时候又会让人看的云里雾里.所以适当的注释同样是有必要的.Python里面的有这么个东西 docstring,使用方法是用三引号给标记出来,python在适当的时候会自动的把这些东西展现出来,比如说,这样:

>>> def my_function():...     """Do nothing, but document it.......     No, really, it doesn‘t do anything....     """...     pass...>>> print my_function.__doc__Do nothing, but document it.    No, really, it doesn‘t do anything.

4.8. Intermezzo: Coding Style

代码风格:每个语言都会有自己的代码风格,Python同样如此.关于代码风格,这里建议看一下PEP8 和PEP20
[pep8]http://legacy.python.org/dev/peps/pep-0008/
[pep20]http://legacy.python.org/dev/peps/pep-0020/
附上原文:

• Use 4-space indentation, and no tabs.

  4 spaces are a good compromise between small indentation (allows greater nesting depth) and large indentation (easier to read). Tabs introduce confusion, and are best left out.

• Wrap lines so that they don’t exceed 79 characters.

  This helps users with small displays and makes it possible to have several code files side-by-side on larger displays.

• Use blank lines to separate functions and classes, and larger blocks of code inside functions.

• When possible, put comments on a line of their own.

• Use docstrings.

• Use spaces around operators and after commas, but not directly inside bracketing constructs: a = f(1, 2) + g(3, 4).

• Name your classes and functions consistently; the convention is to use CamelCase for classes and lower_case_with_underscores for functions and methods. Always use self as the name for the first method argument (see A First Look at Classes for more on classes and methods).

• Don’t use fancy encodings if your code is meant to be used in international environments. Plain ASCII works best in any case.