Python built-in functions reversed, enumerate (continuously updated)

1、reversed

2、enumerate

(1) enumerate() description

• Enumerate means enumeration and enumeration in the dictionary
• For an iterable/traversable object (such as a list, a string), enumerate will form an index sequence, which can be used to get the index and value at the same time
• enumerate is mostly used to get counts in for loops

• For example, for a seq, we get:

  (0, seq[0]), (1, seq[1]), (2, seq[2])

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• enumerate() returns an enumerate object, for example: 
  

(2) use of enumerate

• If you want to traverse both the index and the elements of a list, you can first write this:

list1 = ["这", "是", "一个", "测试"]
for i in range (len(list1)):
    print i ,list1[i]

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• The above method is a bit cumbersome, using enumerate() will be more direct and elegant:

list1 = ["这", "是", "一个", "测试"]
for index, item in enumerate(list1):
    print index, item
>>>
0 这
1 是
2 一个
3 测试

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• enumerate can also receive a second parameter, which is used to specify the starting value of the index, such as:

list1 = ["这", "是", "一个", "测试"]
for index, item in enumerate(list1, 1):
    print index, item
>>>
1 这
2 是
3 一个
4 测试

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(3) Supplement

If you want to count the number of lines in a file, you can write:

count = len(open(filepath, 'r').readlines())

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This method is simple, but can be slow and doesn't even work when the file size is large .

You can use enumerate():

count = 0
for index, line in enumerate(open(filepath,'r'))： 
    count += 1