# 不再写 for 循环

• 在一个序列来提取一些信息。
• 从一个序列生成另一个序列。
• 写 for 已成习惯。

• 较少的代码量
• 更好的代码可读性
• 更少的缩进（对 Python 还是很有意义的）

``````# 1
with ...:
for ...:
if ...:
try:
except:
else:
``````

"扁平结构比嵌套结构更好" - The Zen of Python

### 可以使用的已有的工具来替换 for 循环

1. List Comprehension / Generator 表达式

``````result = []
for item in item_list:
new_item = do_something_with(item)
result.append(item)
``````

``````result = [do_something_with(item) for item in item_list]
``````

``````result = (do_something_with(item) for item in item_list)
``````

2. 函数

``````doubled_list = map(lambda x: x * 2, old_list)
``````

``````from functools import reduce
summation = reduce(lambda x, y: x + y, numbers)
``````

``````>>> a = list(range(10))
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> all(a)
False
>>> any(a)
True
>>> max(a)
9
>>> min(a)
0
>>> list(filter(bool, a))
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> set(a)
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
>>> dict(zip(a,a))
{0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9: 9}
>>> sorted(a, reverse=True)
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> str(a)
'[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]'
>>> sum(a)
45
``````

3. Extract Functions or Generators

``````results = []
for item in item_list:
# setups
# condition
# processing
# calculation
results.append(result)
``````

``````def process_item(item):
# setups
# condition
# processing
# calculation
return result

results = [process_item(item) for item in item_list]
``````

``````results = []
for i in range(10):
for j in range(i):
results.append((i, j))
``````

``````results = [(i, j)
for i in range(10)
for j in range(i)]
``````

``````# finding the max prior to the current item
a = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
results = []
current_max = 0
for i in a:
current_max = max(i, current_max)
results.append(current_max)

# results = [3, 4, 6, 6, 6, 9, 9, 9, 9, 9]
``````

``````def max_generator(numbers):
current_max = 0
for i in numbers:
current_max = max(i, current_max)
yield current_max

a = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
results = list(max_generator(a))
``````

### 不要自己写。 itertools 会帮你实现了

``````from itertools import accumulate
a = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
resutls = list(accumulate(a, max))
``````

### 结论

• 在大多数情况下，您都不需要编写 for 循环。
• 你应该避免编写 for 循环，这样会有更好的代码可读性。