How to perform an assignment inside a lambda function

Question

Python 2.7.1

I would like to understand why I can't do the following which seems like a sensible thing to do

def do_stuff():
    # return a function which takes a map as an argument and puts a key in there
    f = lambda map: map['x'] = 'y' #compilation error
    return f 

x = do_stuff()
map = {}
x(map)
print map['x']

I can have that lambda function to be somethign simpler something like f = lambda map: os.path.exists however I cannot get it to change the map. Can someone tell me how I can achieve this? If this is not possible at all why?

score 15 · Accepted Answer · answered Mar 11 '13 at 14:37

15

You can't use assignment in an expression, it is a statement. A lambda can only contain one expression, and statements are not included.

You can assign to the map though, by using the operator.setitem() function instead:

import operator

lambda map: operator.setitem(map, 'x', 'y')

answered Mar 11 '13 at 14:37

Martijn Pieters

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1

Actually it is cool !! – Kannan Ekanath Mar 11 '13 at 15:53
1

or `map.__setitem__('x', 'y')` – newacct Mar 11 '13 at 20:33
2

@newacct: Yup, but that uses the special hook method directly; better to use an API method. :-) – Martijn Pieters Mar 11 '13 at 20:34

How to perform an assignment inside a lambda function

1 Answers1