How to use filter, map, and reduce in Python 3

问题

filter, map, and reduce work perfectly in Python 2. Here is an example:

>>> def f(x):
        return x % 2 != 0 and x % 3 != 0
>>> filter(f, range(2, 25))
[5, 7, 11, 13, 17, 19, 23]

>>> def cube(x):
        return x*x*x
>>> map(cube, range(1, 11))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]

>>> def add(x,y):
        return x+y
>>> reduce(add, range(1, 11))
55

But in Python 3, I receive the following outputs:

>>> filter(f, range(2, 25))
<filter object at 0x0000000002C14908>

>>> map(cube, range(1, 11))
<map object at 0x0000000002C82B70>

>>> reduce(add, range(1, 11))
Traceback (most recent call last):
  File \"<pyshell#8>\", line 1, in <module>
    reduce(add, range(1, 11))
NameError: name \'reduce\' is not defined

I would appreciate if someone could explain to me why this is.

Screenshot of code for further clarity:

回答1:

You can read about the changes in What's New In Python 3.0. You should read it thoroughly when you move from 2.x to 3.x since a lot has been changed.

The whole answer here are quotes from the documentation.

Views And Iterators Instead Of Lists

Some well-known APIs no longer return lists:

• [...]
• map() and filter() return iterators. If you really need a list, a quick fix is e.g. list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda), or rewriting the code so it doesn’t need a list at all. Particularly tricky is map() invoked for the side effects of the function; the correct transformation is to use a regular for loop (since creating a list would just be wasteful).
• [...]

Builtins

• [...]
• Removed reduce(). Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable.
• [...]

回答2:

The functionality of map and filter was intentionally changed to return iterators, and reduce was removed from being a built-in and placed in functools.reduce.

So, for filter and map, you can wrap them with list() to see the results like you did before.

>>> def f(x): return x % 2 != 0 and x % 3 != 0
...
>>> list(filter(f, range(2, 25)))
[5, 7, 11, 13, 17, 19, 23]
>>> def cube(x): return x*x*x
...
>>> list(map(cube, range(1, 11)))
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
>>> import functools
>>> def add(x,y): return x+y
...
>>> functools.reduce(add, range(1, 11))
55
>>>

The recommendation now is that you replace your usage of map and filter with generators expressions or list comprehensions. Example:

>>> def f(x): return x % 2 != 0 and x % 3 != 0
...
>>> [i for i in range(2, 25) if f(i)]
[5, 7, 11, 13, 17, 19, 23]
>>> def cube(x): return x*x*x
...
>>> [cube(i) for i in range(1, 11)]
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
>>>

They say that for loops are 99 percent of the time easier to read than reduce, but I'd just stick with functools.reduce.

Edit: The 99 percent figure is pulled directly from the What’s New In Python 3.0 page authored by Guido van Rossum.

回答3:

As an addendum to the other answers, this sounds like a fine use-case for a context manager that will re-map the names of these functions to ones which return a list and introduce reduce in the global namespace.

A quick implementation might look like this:

from contextlib import contextmanager    

@contextmanager
def noiters(*funcs):
    if not funcs: 
        funcs = [map, filter, zip] # etc
    from functools import reduce
    globals()[reduce.__name__] = reduce
    for func in funcs:
        globals()[func.__name__] = lambda *ar, func = func, **kwar: list(func(*ar, **kwar))
    try:
        yield
    finally:
        del globals()[reduce.__name__]
        for func in funcs: globals()[func.__name__] = func

With a usage that looks like this:

with noiters(map):
    from operator import add
    print(reduce(add, range(1, 20)))
    print(map(int, ['1', '2']))

Which prints:

190
[1, 2]

Just my 2 cents :-)

回答4:

Since the reduce method has been removed from the built in function from Python3, don't forget to import the functools in your code. Please look at the code snippet below.

import functools
my_list = [10,15,20,25,35]
sum_numbers = functools.reduce(lambda x ,y : x+y , my_list)
print(sum_numbers)

回答5:

Here are the examples of Filter, map and reduce functions.

numbers = [10,11,12,22,34,43,54,34,67,87,88,98,99,87,44,66]

//Filter

oddNumbers = list(filter(lambda x: x%2 != 0, numbers))

print(oddNumbers)

//Map

multiplyOf2 = list(map(lambda x: x*2, numbers))

print(multiplyOf2)

//Reduce

The reduce function, since it is not commonly used, was removed from the built-in functions in Python 3. It is still available in the functools module, so you can do:

from functools import reduce

sumOfNumbers = reduce(lambda x,y: x+y, numbers)

print(sumOfNumbers)

来源：https://stackoverflow.com/questions/13638898/how-to-use-filter-map-and-reduce-in-python-3