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Python支持运行时使用“lambda”建立匿名函数（anonymous functions that are not bound to a name)。
python "lambda"和functional programming语言有区别，但是他非常强大经常拿来和诸如filter(),map(),reduce()
等经典概念结合。
以下示例普通函数和匿名函数：
1 In [113]: def normalFun (x): return x**2 2 3 In [114]: print normalFun(8) 4 64 5 6 In [115]: anonymousFun = lambda x:x**2 7 8 In [116]: print anonymousFun(8) 9 64
普通函数和匿名函数运算结果都一样，但是匿名函数没有return语句，冒号后边
表达式就是返回值。
2 下面代码片段展示匿名函数用法，请保证python版本在2.2以上，因为需要支持嵌入作用域。
In [120]: def make_incrementor (n): return lambda x: x + n In [121]: f = make_incrementor(2) In [122]: g = make_incrementor(6) In [123]: f Out[123]: <function __main__.<lambda>> In [124]: g Out[124]: <function __main__.<lambda>> In [125]: print(42) 42 In [126]: print f(42) 44 In [127]: print g(42) 48
注意，g,f 定义后类型显示位<function __main__.<lambda>>，说明此时g,f是匿名函数。
3 以下几个代码示范lambda和其他结合用法
In [133]: foo = [2, 18, 9, 22, 17, 24, 8, 12, 27] In [134]: print filter(lambda x: x % 3 == 0, foo) [18, 9, 24, 12, 27] In [135]: ?filter Type: builtin_function_or_method String form: <built-in function filter> Namespace: Python builtin Docstring: filter(function or None, sequence) -> list, tuple, or string Return those items of sequence for which function(item) is true. If function is None, return the items that are true. If sequence is a tuple or string, return the same type, else return a list. In [136]: print filter(foo) --------------------------------------------------------------------------- TypeError Traceback (most recent call last) <ipython-input-136-a3d764429161> in <module>() ----> 1 print filter(foo) TypeError: filter expected 2 arguments, got 1 In [137]: print filter(None,foo) [2, 18, 9, 22, 17, 24, 8, 12, 27]
?map Type: builtin_function_or_method String form: <built-in function map> Namespace: Python builtin Docstring: map(function, sequence[, sequence, ...]) -> list Return a list of the results of applying the function to the items of the argument sequence(s). If more than one sequence is given, the function is called with an argument list consisting of the corresponding item of each sequence, substituting None for missing values when not all sequences have the same length. If the function is None, return a list of the items of the sequence (or a list of tuples if more than one sequence). In [140]: print map(lambda x: x* 2 + 100, foo) [104, 136, 118, 144, 134, 148, 116, 124, 154] In [141]: foo Out[141]: [2, 18, 9, 22, 17, 24, 8, 12, 27] In [142]:
In [146]: print reduce(lambda x,y: x + y, foo) 139 In [147]: ?reduce Type: builtin_function_or_method String form: <built-in function reduce> Namespace: Python builtin Docstring: reduce(function, sequence[, initial]) -> value Apply a function of two arguments cumulatively to the items of a sequence, from left to right, so as to reduce the sequence to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). If initial is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty. In [148]: foo Out[148]: [2, 18, 9, 22, 17, 24, 8, 12, 27]
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原文地址：http://www.cnblogs.com/wenwangt/p/4467277.html
