I want to understand the concept of list comprehension in python

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Given code
x=[(a,b) for a in range(3) for b in range(a)]
print(x)
Output
[(1, 0), (2, 0), (2, 1)] 

I know the comprehension can be broken down as follows:

for a in range(3):
    for b in range(a):
        print(a,b)

Can someone explain me how the output came as such provided above....


I think what you're looking for is this:

  1. first, a=0 so the inner loop skips as range(0) returns an empty list.
  2. then, a=1 so the inner loop is only executed for 0 as range(1) is qual to [0] so the first pair is (1, 0)
  3. a=2 and the inner loop runs for b=0 and b=1 hence the other two pair (2, 0) and (2, 1)

I think it's clear what's going on now. right?

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The list comprehension is not translated to:

for a in range(3):
    for b in range(a):
        print(a,b)

but rather to:

x = []
for a in range(3):
    for b in range(a):
        x.append((a, b))

(Tutorial) Python List Comprehension, Learn how to effectively use list comprehension in Python to create lists, For example, let's say you want to build a list of courses then you Consider the following example that starts with the variable numbers , defined as a� List Comprehension is one of the most powerful features available in dealing with Python lists. This feature is already at your fingertips. No need to import any library or modules, since list…


You are running in parallel range(3) and range(a).

What effectively happens is that a is being set to 0, 1 and finally 2.

Then, your b is defined for each element in range(a).

Now data is generated:

a = 0
range(0) # nothing
a = 1
range(1) # (0, )
(1, 0) # first pair of (a, b) for a = 1
a = 2
range(2) # (0, 1)
(2, 0) # first pair of (a, b) for a = 2
(2, 1) # second pair of (a, b) for a = 2
# end

Given the above, result ends up being [(1, 0), (2, 0), (2, 1)]

Comprehending the 'Comprehensions' in Python, Understanding and implementing the list, dictionary, set, and generator Python 2.0 introduced us to the concept of list comprehensions, and Python 3.0 took it Say, we need to find the squares of the first five even numbers. List comprehension is an elegant way to define and create lists based on existing lists. List comprehension is generally more compact and faster than normal. a = [i for i in range(1, 1000) if x % 7


what the list comprehension does is this:

ret = []
for a in range(3):
    for b in range(a):
        ret.append((a,b))

print(ret)  # [(1, 0), (2, 0), (2, 1)] 

which returns the list you are getting...

Python List Comprehensions: Explained Visually, The key to understanding when to use list comprehensions is to practice You can rewrite the above for loop as a list comprehension like this:� In Python, list comprehensions are constructed like so: list_variable = [x for x in iterable] A list, or other iterable, is assigned to a variable. Additional variables that stand for items within the iterable are constructed around a for clause.


List Comprehensions in Python, The expressions can be anything, meaning you can put in all kinds Now when we know the syntax of list comprehensions, let's show some examples and For the next example, assume we want to create a list of squares. Python Advanced List Methods with Examples: In this tutorial, we will explore some of the Advanced concepts in Python list. The concepts in Python Advanced list includes Python Sort Method, Sorted function, Python Reverse List, Python Index Method, Copying a List, Python Join Function, Sum Function, Removing duplicates from the List, Python List Comprehension, etc.


In terms of performance in Python, is a list-comprehension, or functions like map(), filter()and reduce()faster than a for loop? Why, technically, they run in a C speed, while the for loop runs in the python virtual machine speed?. Suppose that in a game that I'm developing I need to draw complex and huge maps using for loops.


In Python, you have heard that lists, strings and tuples are ordered collection of objects and sets and dictionaries are unordered collection of objects. So, do you understand what are ordered and unordered collection of objects in Python? If you don't then following example helps you to understand concept ordered vs unordered: