Question

我对编程很陌生，并且不了解很多概念。有人可以向我解释第2行的语法及其工作原理吗？是否需要缩进？而且，我可以从哪里学到这些？

string = #extremely large number

num = [int(c) for c in string if not c.isspace()]

Answer 1

这是list comprehension，是一种创建新列表的简写。它在功能上等同于：

num = []
for c in string:
    if not c.isspace():
       num.append(int(c))

Answer 2

这正是它所说的。

num   =        [          int(                      c)

"num" shall be a list of: the int created from each c

for   c                           in string

where c takes on each value found in string

if        not                     c .isspace() ]

such that it is not the case that c is a space (end of list description)

Answer 3

只是为了扩展mgilson的答案，好像你是一个相当新的编程，这也可能有点迟钝。几个月前我开始学习python，这是我的注释。

string = 'aVeryLargeNumber'
num = [int(c) for c in string if not c.isspace()] #list comprehension

"""Breakdown of a list comprehension into it's parts."""
num = [] #creates an empty list
for c in string: #This threw me for a loop when I first started learning
     #as everytime I ran into the 'for something in somethingelse':
     #the c was always something else. The c is just a place holder
     #for a smaller unit in the string (in this example). 
     #For instance we could also write it as:
     #for number in '1234567890':, which is also equivalent to 
     #for x in '1234567890': or 
     #for whatever in '1234567890'
             #Typically you want to use something descriptive.
     #Also, string, does not have to be just a string. It can be anything
     #so long as you can iterate (go through it) one item at a time
     #such as a list, tuple, dictionary.

if not c.isspace(): #in this example it means if c is not a whitespace character
        #which is a space, line feed, carriage return, form feed,
                #horizontal tab, vertical tab.

num.append(int(c))  #This converts the string representation of a number to an actual
        #number(technically an integer), and appends it to a list. 

'1234567890' # our string in this example
num = []
    for c in '1234567890':
        if not c.isspace():
            num.append(int(c))

循环的第一次迭代看起来像：

num = [] #our list, empty for now
    for '1' in '1234567890':
        if not '1'.isspace():
            num.append(int('1'))

注意''围绕1.'之间的任何内容''或“”表示此项目是一个字符串。虽然它看起来像一个数字，但就Python而言它并非如此。一个简单的方法验证是在解释器中键入1 + 2并将结果与'1'+'2'进行比较。看到差异？使用数字，它可以按照您的预期将它们添加到一起。使用字符串将它们连接在一起。

到第二遍！

num = [1] #our list, now with a one appended!
    for '2' in '1234567890':
        if not '2'.isspace():
            num.append(int('2'))

所以它会一直持续到字符串中的字符用完为止，否则会产生错误。如果字符串是'1234567890.12345'会发生什么？我们可以有把握地说''。不是空白字符。因此，当我们进入int（'。'）时，Python会抛出一个错误：

Traceback (most recent call last):
    File "<string>", line 1, in <fragment>
builtins.ValueError: invalid literal for int() with base 10: '.'

就学习Python的资源而言，有很多免费教程，例如：

http://www.learnpython.org

http://learnpythonthehardway.org/book

http://openbookproject.net/thinkcs/python/english3e

http://getpython3.com/diveintopython3

如果你想买一本学习书，那么：http://www.amazon.com/Learning-Python-Powerful-Object-Oriented-Programming/dp/0596158068是我最喜欢的。不确定为什么评级很低但我认为作者做得很好。

祝你好运！

Answer 4

PEP中的这些示例是一个很好的起点。如果您不熟悉range和%，则需要退后一步，了解有关基金会的更多信息。

>>> print [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> print [i for i in range(20) if i%2 == 0]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

>>> nums = [1,2,3,4]
>>> fruit = ["Apples", "Peaches", "Pears", "Bananas"]
>>> print [(i,f) for i in nums for f in fruit]
[(1, 'Apples'), (1, 'Peaches'), (1, 'Pears'), (1, 'Bananas'),
 (2, 'Apples'), (2, 'Peaches'), (2, 'Pears'), (2, 'Bananas'),
 (3, 'Apples'), (3, 'Peaches'), (3, 'Pears'), (3, 'Bananas'),
 (4, 'Apples'), (4, 'Peaches'), (4, 'Pears'), (4, 'Bananas')]
>>> print [(i,f) for i in nums for f in fruit if f[0] == "P"]
[(1, 'Peaches'), (1, 'Pears'),
 (2, 'Peaches'), (2, 'Pears'),
 (3, 'Peaches'), (3, 'Pears'),
 (4, 'Peaches'), (4, 'Pears')]
>>> print [(i,f) for i in nums for f in fruit if f[0] == "P" if i%2 == 1]
[(1, 'Peaches'), (1, 'Pears'), (3, 'Peaches'), (3, 'Pears')]
>>> print [i for i in zip(nums,fruit) if i[0]%2==0]

复杂的语法 - Python

4 个答案: