How to use the Python list loop statement

One of the powerful features of Python is its parsing of the list, which provides a compact way to map a list to another list by applying a function to each element in the list.

Instance

A = [' Cat ', ' window ', ' defenestrate ']
For X in a:
Print x, Len (x)

For x in [1, 2, 3]: print x, # iteration

Instance

Loop through a list:for in

A = [' Cat ', ' window ', ' defenestrate ']

For x in a[:]: # Make a slice copy of the entire list
If Len (x) > 6:a.insert (0, X)

Print a

Operation based on array length

A = [' Mary ', ' had ', ' a ', ' little ', ' lamb ']

For I in range (Len (a)):
Print I, A[i]

Instance

words = [' A ', ' B ', ' C ', ' D ', ' E ']
For word in words:
Print Word

3.24. List resolution Introduction

>>> li = [1, 9, 8, 4]
>>> [elem*2 for Elem in Li]
[2, 18, 16, 8]
>>> Li
[1, 9, 8, 4]
>>> Li = [elem*2 for Elem in Li]
>>> Li
[2, 18, 16, 8]

To make it easier to understand, let's look at it from right to left. Li is a list that will be mapped. Python loops through each element in the LI. For each element, you first assign the value to the variable elem, and then Python applies the function elem*2 to the calculation, and then appends the result to the list to be returned.

It should be noted that the parsing of the list does not change the original list.

It is safe to assign the parse result of a list to the variable to which it is mapped. There's no need to worry about competition or any weird thing happening. Python creates a new list in memory, and Python assigns the result to a variable when parsing the list is complete.

From <dive into python>