我在试着找出Python的lambda。lambda是那些在现实生活中应该被遗忘的“有趣”语言项之一吗?
我确信有一些边缘情况可能需要它,但是考虑到它的模糊性,它在未来版本中被重新定义的可能性(我基于它的各种定义的假设)以及降低的编码清晰度——应该避免它吗?
这让我想起了C类型的溢出(缓冲区溢出)——指向顶部变量并重载以设置其他字段值。这感觉有点像技术人员的表演,但维护程序员的噩梦。
我在试着找出Python的lambda。lambda是那些在现实生活中应该被遗忘的“有趣”语言项之一吗?
我确信有一些边缘情况可能需要它,但是考虑到它的模糊性,它在未来版本中被重新定义的可能性(我基于它的各种定义的假设)以及降低的编码清晰度——应该避免它吗?
这让我想起了C类型的溢出(缓冲区溢出)——指向顶部变量并重载以设置其他字段值。这感觉有点像技术人员的表演,但维护程序员的噩梦。
当前回答
我经常使用它,主要是作为空对象或将参数部分绑定到函数。
下面是一些例子:
实现空对象模式:
{
DATA_PACKET: self.handle_data_packets
NET_PACKET: self.handle_hardware_packets
}.get(packet_type, lambda x : None)(payload)
对于参数绑定:
假设我有以下API
def dump_hex(file, var)
# some code
pass
class X(object):
#...
def packet_received(data):
# some kind of preprocessing
self.callback(data)
#...
然后,当我不想快速转储接收到的数据到一个文件,我这样做:
dump_file = file('hex_dump.txt','w')
X.callback = lambda (x): dump_hex(dump_file, x)
...
dump_file.close()
其他回答
只是函数的一种奇特说法。除了它的名字,它没有什么晦涩、吓人或神秘的地方。当你读到下面这行,在脑海中用函数替换lambda:
>>> f = lambda x: x + 1
>>> f(3)
4
它只是定义了一个关于x的函数。其他一些语言,比如R,显式地说:
> f = function(x) { x + 1 }
> f(3)
4
你看到了什么?这是编程中最自然的事情之一。
你可以用lambda做的任何事情,都可以用命名函数或列表和生成器表达式做得更好。
因此,在大多数情况下,在任何情况下您都应该只使用其中一种(可能除了在交互式解释器中编写的草稿代码)。
我使用lambda来创建包含参数的回调。在一行中编写lambda比编写一个方法来执行相同的功能更简洁。
例如:
import imported.module
def func():
return lambda: imported.module.method("foo", "bar")
相对于:
import imported.module
def func():
def cb():
return imported.module.method("foo", "bar")
return cb
我是一个python初学者,所以为了获得一个清晰的lambda概念,我将它与'for'循环进行了比较;在效率方面。 下面是代码(python 2.7) -
import time
start = time.time() # Measure the time taken for execution
def first():
squares = map(lambda x: x**2, range(10))
# ^ Lambda
end = time.time()
elapsed = end - start
print elapsed + ' seconds'
return elapsed # gives 0.0 seconds
def second():
lst = []
for i in range(10):
lst.append(i**2)
# ^ a 'for' loop
end = time.time()
elapsed = end - start
print elapsed + ' seconds'
return elapsed # gives 0.0019998550415 seconds.
print abs(second() - first()) # Gives 0.0019998550415 seconds!(duh)
I started reading David Mertz's book today 'Text Processing in Python.' While he has a fairly terse description of Lambda's the examples in the first chapter combined with the explanation in Appendix A made them jump off the page for me (finally) and all of a sudden I understood their value. That is not to say his explanation will work for you and I am still at the discovery stage so I will not attempt to add to these responses other than the following: I am new to Python I am new to OOP Lambdas were a struggle for me Now that I read Mertz, I think I get them and I see them as very useful as I think they allow a cleaner approach to programming.
He reproduces the Zen of Python, one line of which is Simple is better than complex. As a non-OOP programmer reading code with lambdas (and until last week list comprehensions) I have thought-This is simple?. I finally realized today that actually these features make the code much more readable, and understandable than the alternative-which is invariably a loop of some sort. I also realized that like financial statements-Python was not designed for the novice user, rather it is designed for the user that wants to get educated. I can't believe how powerful this language is. When it dawned on me (finally) the purpose and value of lambdas I wanted to rip up about 30 programs and start over putting in lambdas where appropriate.