List Comprehension : 

 List comprehension is used to create an iterable which follows a particular fashion.

List comprehensions are a syntactic sugar for creating a list(iterable).

>>>"Expected output : [0, 0, 0, 0, 0]"
>>> 'Native way : '
>>> list = []
>>> for i in range(5):
...     list.append(0)
...
>>> list
[0, 0, 0, 0, 0]

"Using list comprehension : "

>>> list = [0 for i in range(5)]
>>> list
[0, 0, 0, 0, 0]

Isn't it fun and easy to read and write??

Let's understand it.

list = [0 for i in range(5)]

Here, there is a loop counter i which takes value from 0, 4 using the range function and foreverey value of the loop counter, 

0 is appended to the list.

Another example:

Creating [0, 1, 2, 3, 4, 5] : 

In previous example, we had appended 0 for each value of loop counter, 

Now, we will append the loop counter value:

>>> list = [i for i in range(5)]
>>> list
[0, 1, 2, 3, 4]

 Note : this can be achieved using list(range(5)), but for example i did this in that way.

just like normal for loop, we can iterate over an iterable, same can be done in list comprehension.

For example : 

1)

a = [1, 4, 7, 2]

b = [i+1 for i in a]

>>> a = [1, 4, 7, 2]
>>> b = [i+1 for i in a]
>>> b
[2, 5, 8, 3]

2)

 >>> names = ['math', 'pyqt', 'numpy', 'matplotlib', 'keras', 'tensorflow']
>>> length = [len(name) for name in names]
>>> length
[4, 4, 5, 10, 5, 10]

3)

>>> numbers = [2, 5, 1, 9]
>>> squares = [n*n for n in numbers]
>>> squares
[4, 25, 1, 81]

Easy, correct??

You may have a doubt that, There are some conditions where the list elements are not following same output which is dependent on conditions.

List comprehension supports if else (any boolean) conditions.

Case 1) Only if condition is needed.

Structure:

[operation for something in something  if condition]

>>> numbers = [3, 2, 1, 4, 5, 7, 6]
>>> even = [n for n in numbers if n%2==0]
>>> even
[2, 4, 6]

Case 2) Both if and else condition is needed.

Structure:

[cto if cond else fco for iteratioon]

cond :  condition to be checked

cto : condition true operation

fco : condition false operation.

Example:

>>> numbers = [3, 2, 1, 4, 5, 7, 6]
>>> even = [1 if n%2 == 0 else 0 for n in numbers]
>>> even
[0, 1, 0, 1, 0, 0, 1]

Nested List comprehension : 

It is generally used in representing graphs and multi-dimensional datatypes.

for example:

1)

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

 can be created using:

 >>> [[0 for i in range(4)] for i in range(4)]
[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

2)

0 1 2 3

0 1 2 3

0 1 2 3

0 1 2 3

>>> [[i for i in range(4)] for i in range(4)]
[[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

3)

Problem : Create a 2d list whose (i ,j)th element is (i+j)

Soln:

0 1 2 3 

1 2 3 4

2 3 4 5

3 4 5 6

>>> [[i+j for i in range(4)] for j in range(4)]
[[0, 1, 2, 3], [1, 2, 3, 4], [2, 3, 4, 5], [3, 4, 5, 6]]

4)

In Engineering / related courses, we generally come across situations for initializing with some conditions like:

(i, j)th element   =    0            if i = j;

                                 (i+j)!     otherwise

Soln:

0   1  2     6

1   0  6     24

2   6  0     120

6  24 120 0

>>> [[factorial(i+j) if i !=j else 0 for i in range(4)] for j in range(4)]
[[0, 1, 2, 6], [1, 0, 6, 24], [2, 6, 0, 120], [6, 24, 120, 0]]

Note : List comprehensions are not only used to create a list. ;)

A generator object, set, dictionary ....... any iterable,
except for tuples.

>>> (i for i in range(5))
<generator object <genexpr> at 0x7f654145a4c0>

This is what happens when you try to create a tuple.
Although, type casting does the task.

Sets : 

>>> {2*i for i in range(7)}
{0, 2, 4, 6, 8, 10, 12}

Dictionary:

>>> cubes = {i : i*i*i  for i in range(7)}
>>> cubes
{0: 0, 1: 1, 2: 8, 3: 27, 4: 64, 5: 125, 6: 216}