When to Use List Comprehensions

Good presentation of list comprehension, helped me a lot understand the concept and the execution sequence.

Thanks Eriberto, that’s great to hear! We’ve got some more list comprehension tips in our upcoming Python Interview Tips article.

The Python Tricks book also has a chapter on them if you’re looking to dig deeper :)

HI,

How is list comprehension different from map() function? They seem to do the same functionality.

List Comprehension vs map() in Python: How is a list comprehension different from the map() function? They seem to do the same functionality.

Great question! I’m going to quote a comment thread from python-guide.org where this was discussed:

PEP 202 says:

List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

Source: www.python.org/dev/peps/pep-0202/

Interestingly Guido van Rossum (the creator of Python) says using map with lambda as one of his “regrets” and to use a list comprehension instead. Then in the very next section he says a for loop is more clear than reduce().

• map(), filter()
• using a Python function here is slow

• list comprehensions do the same thing better

• reduce()

• nobody uses it, few understand it
• a for loop is clearer & (usually) faster

Source: www.python.org/doc/essays/ppt/regrets/PythonRegrets.pdf

Also Guido said:

“It has been argued that the real problem here is that Python’s lambda notation is too verbose, and that a more concise notation for anonymous functions would make map() more attractive. Personally, I disagree—I find the list comprehension notation much easier to read than the functional notation, especially as the complexity of the expression to be mapped increases. In addition, the list comprehension executes much faster than the solution using map and lambda. This is because calling a lambda function creates a new stack frame while the expression in the list comprehension is evaluated without creating a new stack frame.”

Source: python-history.blogspot.com/2010/06/from-list-comprehensions-to-generator.html

There’s also a performance difference, please see stackoverflow.com/questions/1247486/python-list-comprehension-vs-map

Could you please link to or make a video on n-dimensional lists, and list comprehension with lambda.

good video, thank you

The formula for the general list comprehension with condition and expression is a helpful abstraction.

Very good presentation; thanks!

IMHO, if your list comprehension has to be broken up into multiple lines due to its length, then I don’t see the advantage of using it over a for loop anymore.

@skrabbit9000 I agree with that. For long list comprehensions it’s usually better to stick with for loops instead.

Thank you for putting this together. It helped me to better understand what goes into the decision of when to use a list comprehension.

Thanks for the great, quick overview of list comprehensions.

Nice presentation, also fully agreed with, avoid to use nested list comprehension into your code. Better to go for loops.. Thanks Dan for nicely compiled the course.

Great break down with the generalized templates there, its a solid way to imprint the syntax in the brain. I have used list comprehensions a lot in projects and found that proper naming (especially the variable for each item) really helped with those lengthy and hard to read comprehensions. An example from the one used on the course can be seen below:

def is_even(x: int) -> bool:
    return x % 2 == 0

def get_squared_value(x: int) -> int:
    return x**2

even_squares = [get_squared_value(number) 
                for number in range(10) 
                if is_even(number)
]

Thanks for explaining this concept.

It’s fun how after just seeing a few examples and understanding how a list comprehension is “set up”, you suddenly can read more code.

I was following a different lesson, about api integrations here and was having trouble understanding this syntax:

max(country["id"] for country in countries) + 1

But now I understand… I think.

Although, the example above is iterating through an object and not a list. But guess the principle is the same.

I would’ve loved to see something like this being handled in this lesson.

Very comprehensive lesson on list comprehensions. Thank you, Dan!