Python Language
- Getting started with Python Language
- Awesome Book
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- *args and **kwargs
- 2to3 tool
- Abstract Base Classes (abc)
- Abstract syntax tree
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- Introduction to RabbitMQ using AMQPStorm
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- JSON Module
- kivy - Cross-platform Python Framework for NUI Development
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- List comprehensions
- List Comprehensions
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- Iterate two or more list simultaneously within list comprehension
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- Nested List Comprehensions
- Refactoring filter and map to list comprehensions
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- Writing to CSV from String or List
Python Language List Comprehensions Nested List Comprehensions
Example
Nested list comprehensions, unlike list comprehensions with nested loops, are List comprehensions within a list comprehension. The initial expression can be any arbitrary expression, including another list comprehension.
#List Comprehension with nested loop
[x + y for x in [1, 2, 3] for y in [3, 4, 5]]
#Out: [4, 5, 6, 5, 6, 7, 6, 7, 8]
#Nested List Comprehension
[[x + y for x in [1, 2, 3]] for y in [3, 4, 5]]
#Out: [[4, 5, 6], [5, 6, 7], [6, 7, 8]]
The Nested example is equivalent to
l = []
for y in [3, 4, 5]:
temp = []
for x in [1, 2, 3]:
temp.append(x + y)
l.append(temp)
One example where a nested comprehension can be used it to transpose a matrix.
matrix = [[1,2,3],
[4,5,6],
[7,8,9]]
[[row[i] for row in matrix] for i in range(len(matrix))]
# [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
Like nested for loops, there is not limit to how deep comprehensions can be nested.
[[[i + j + k for k in 'cd'] for j in 'ab'] for i in '12']
# Out: [[['1ac', '1ad'], ['1bc', '1bd']], [['2ac', '2ad'], ['2bc', '2bd']]]
