使用Python的filter与map函数后得到空列表的原因排查

Hey there! Let's break down why your code is ending up with an empty list, and fix it step by step.

The Root Cause #

First, let's zero in on the critical mistake in your code:

b = filter(even, a)
print(list(b))  # This line drains the iterator completely
c = map(cube, b)
print(list(c))  # Now b has no elements left to process!

In Python, filter() returns an iterator—not a static list. Iterators are one-time-use objects: once you traverse them (like converting to a list with list(b)), they're empty forever. When you later pass b to map(), there's nothing left to work with, hence the empty final list.

Fix 1: Avoid Exhausting the Iterator Early #

If you don't need to print the filtered values first, skip converting b to a list until after you've used it with map():

# Program to map and filter to make a list whose elements are cubes of even numbers
def even(num):
    # Explicitly return a boolean (non-zero values work, but this makes your intent clearer)
    return num % 2 == 0

def cube(num):
    return num ** 3

a = range(1, 10)
print(list(a))  # Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

b = filter(even, a)
c = map(cube, b)
print(list(c))  # Output: [8, 64, 216, 512]

Fix 2: Convert Filter Result to a Reusable List #

If you want to inspect the filtered values before mapping, convert b to a list right away. Lists are reusable, unlike iterators:

b = list(filter(even, a))
print(b)  # Output: [2, 4, 6, 8]
c = map(cube, b)
print(list(c))  # Output: [8, 64, 216, 512]

Bonus: Simplify with Lambda Expressions #

You can streamline your code even further by using lambda functions instead of defining separate even and cube functions:

a = range(1, 10)
cubes_of_evens = list(map(lambda x: x**3, filter(lambda x: x%2 == 0, a)))
print(cubes_of_evens)  # Output: [8, 64, 216, 512]

内容的提问来源于stack exchange，提问作者Shahbaj