Python List Comprehensions

Notes for the article Python List Comprehensions: Explained Visually by Trey Hunner

Making a List

Integrated with loops



In [1]:

    
list_with_for_loop = [x for x in range(10)]
print list_with_for_loop









    



[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Even with conditions in the for loop



In [2]:

    
list_with_for_loop_conditional = [x for x in range(10) if x%2 == 1]
print list_with_for_loop_conditional









    



[1, 3, 5, 7, 9]

Nested loops in a list



In [3]:

    
list_with_nested_loops = [ [x, y] for x in range(3) for y in range(3) ]
print list_with_nested_loops









    



[[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]]

Another example of nested loops



In [4]:

    
list_with_nested_loops_2 = [ x for x in range(y) for y in range(3)]
print list_with_nested_loops_2









    



[0, 0, 0, 1, 1, 1]

The article gives an example of how to flatten a matrix using this trick. Semantically, one would using



In [5]:

    
matrix = [[11,12],[21,22]]
row = [1,2]
wrong_flatten_of_matrix = [x for x in row for row in matrix]
print "matrix is", matrix
print "flattened matrix is", wrong_flatten_of_matrix









    



matrix is [[11, 12], [21, 22]]
flattened matrix is [1, 1, 2, 2]

which is obviously WRONG. The correct code is given by the author as



In [6]:

    
right_flatten_of_matrix = [x for row in matrix for x in row]
print "matrix is", matrix
print "flattened matrix is", right_flatten_of_matrix









    



matrix is [[11, 12], [21, 22]]
flattened matrix is [11, 12, 21, 22]

The key is to write the nested loops in a list as the normal nested loops.

With this possible confusion, the author proposed a line breaking solution



In [7]:

    
right_flatten_of_matrix_line_breaking = [
    x 
    for row in matrix 
        for x in row
]
print "matrix is", matrix
print "flattened matrix is", right_flatten_of_matrix_line_breaking









    



matrix is [[11, 12], [21, 22]]
flattened matrix is [11, 12, 21, 22]

which significantly improved the readability.