Python lambda function declaration and call for list generation

Question

In generating a list as below, the lambda function is declared outside the squared brackets, then applied for each i,

>>> g = lambda x: x*10
>>> [g(i) for i in range(10,21)]
[100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200]

Is it possible (and how) to declare the lambda function within the list and call it ?

First guess: [(lambda x: x*10)(i) for i in range(10,21)]. Why do you feel you want to do this? — vaultah
– vaultah, Commented Jan 9, 2015 at 20:26
@enzyme Be careful, short code does not always mean efficient code in Python. — augurar
– augurar, Commented Jan 9, 2015 at 20:34
you would never ever want to do this. a lambda is just a one-line function and you can easily just put its body inside the list comprehension. — acushner
– acushner, Commented Jan 9, 2015 at 20:36
Assigning a lambda to a variable is rarely correct but a bunch of horrible lambda tutorials do it anyway. — tdelaney
– tdelaney, Commented Jan 9, 2015 at 21:00

Ignacio Vazquez-Abrams · Accepted Answer · 2015-01-09 20:27:26Z

5

The insane way:

[(lambda x: x*10)(i) for i in range(10,21)]

The sane way:

[i * 10 for i in range(10,21)]

answered Jan 9, 2015 at 20:27

Ignacio Vazquez-Abrams

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1 Comment

augurar Over a year ago

Explanation: The problem with declaring the lambda function in the comprehension is that the function will be repeatedly created for each item in the list. Since the body of a lambda is a single expression, it is better to put that expression directly into the comprehension.

Hackaholic · Accepted Answer · 2015-01-09 20:29:01Z

3

you can use map too:

>>> list(map(lambda x:x*10,range(10,21)))
[100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200]

answered Jan 9, 2015 at 20:29

Hackaholic

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4 Comments

tdelaney Over a year ago

Great answer except that map returns a list so doesn't need to be converted.

tdelaney Over a year ago

oops, should have thought about that. the comment is true for python 2.

Hackaholic Over a year ago

Grt by urself you realize it :)

augurar Over a year ago

@tdelaney Python 2 version: map(lambda x:x*10, xrange(10,21)).

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