The derivation expression is actually to simplify some loop judgment operations, etc

How many ways can you generate a list of numbers 1-10?

>>> l = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] >>> l [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>>

>>> l = [] >>> for x in range( 1, 11 ): ... l.append( x ) ... >>> l [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>>

>>> l = range( 1, 11 ) >>> l [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>>

Now let's look at the derivation

>>> a = [ x for x in range( 1, 11 ) ] >>> a [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>>

Some people may say that it's better to directly range (1, 11). It's unnecessary. If we want to filter out odd numbers?

Of course, range can still:

>>> range( 1, 11, 2 ) [1, 3, 5, 7, 9] >>>

So, if we want to get even numbers, we need to go through every term, judge

>>> a = [] >>> for x in range( 1, 11 ): ... if x % 2 == 0: ... a.append( x ) ... >>> >>> a [2, 4, 6, 8, 10] >>>

He is equivalent to the following derivation:

>>> b = [ x for x in range( 1, 11 ) if x % 2 == 0 ] >>> b [2, 4, 6, 8, 10] >>>

In a word

Generate a coordinate system?

>>> dot = [(x,y) for x in range( 1, 10 ) for y in range( 1, 10 ) ] >>> dot [(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (1, 9), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (2, 7), (2, 8), (2, 9), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8), (3, 9), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (4, 6), (4, 7), (4, 8), (4, 9), (5, 1), (5, 2), (5, 3), (5, 4), (5, 5), (5, 6), (5, 7), (5, 8), (5, 9), (6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (6, 6), (6, 7), (6, 8), (6, 9), (7, 1), (7, 2), (7, 3), (7, 4), (7, 5), (7, 6), (7, 7), (7, 8), (7, 9), (8, 1), (8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7), (8, 8), (8, 9), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8), (9, 9)] >>>

Equivalent to the following 2-fold cycle:

>>> dot = [] >>> for x in range( 1, 10 ): ... for y in range( 1, 10 ): ... dot.append( ( x, y ) ) ... >>> dot [(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6), (1, 7), (1, 8), (1, 9), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6), (2, 7), (2, 8), (2, 9), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8), (3, 9), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (4, 6), (4, 7), (4, 8), (4, 9), (5, 1), (5, 2), (5, 3), (5, 4), (5, 5), (5, 6), (5, 7), (5, 8), (5, 9), (6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (6, 6), (6, 7), (6, 8), (6, 9), (7, 1), (7, 2), (7, 3), (7, 4), (7, 5), (7, 6), (7, 7), (7, 8), (7, 9), (8, 1), (8, 2), (8, 3), (8, 4), (8, 5), (8, 6), (8, 7), (8, 8), (8, 9), (9, 1), (9, 2), (9, 3), (9, 4), (9, 5), (9, 6), (9, 7), (9, 8), (9, 9)] >>>

Generate an incremental string list:

>>> ['the number:%s' % n for n in range( 1, 10 ) ] ['the number:1', 'the number:2', 'the number:3', 'the number:4', 'the number:5', 'the number:6', 'the number:7', 'the number:8', 'the number:9']

Find the square of 1-9

>>> [x ** 2 for x in range( 1, 10 ) ] [1, 4, 9, 16, 25, 36, 49, 64, 81] >>>

Use the dictionary to package a layer. The same key will cover the previous one

>>> dict( [( x, y ) for x in range( 1, 5 ) for y in range( 1, 5 )] ) {1: 4, 2: 4, 3: 4, 4: 4} >>> [( x, y ) for x in range( 1, 5 ) for y in range( 1, 5 )] [(1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3), (2, 4), (3, 1), (3, 2), (3, 3), (3, 4), (4, 1), (4, 2), (4, 3), (4, 4)]

List reference, and javascript type:

>>> l = ['my', 'name', 'is', 'ghostwu' ] >>> l ['my', 'name', 'is', 'ghostwu'] >>> a = l >>> a[3] = 'wukong' >>> l ['my', 'name', 'is', 'wukong'] >>> del a >>> l ['my', 'name', 'is', 'wukong'] >>> a Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'a' is not defined >>>

del a, delete the list reference, similar to the php garbage collection mechanism, two variables point to a list, delete one, but the other still points to that list

>>> a = [10, 20, 30 ] >>> b = a >>> b [10, 20, 30] >>> del a >>> a Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'a' is not defined >>> b [10, 20, 30] >>>

del a [], which means clear list

>>> a = [ 10, 20, 30 ] >>> b = a >>> del a[:] >>> >>> a [] >>> b [] >>>