How to assign values in lambda function in Python?

Question

I am using lambda function to make my code optimize and faster. Below I have written some function.

a = [{"objId":"5c077187fe506f8dd3589ce6","userid":"absurana","firstName":"Null","usrRole":"Software Quality User","lastName":"Null","tiles":"Potential CFD","userType":"User"},
 {"objId":"5d9d7ce6fe506f11b275d01b","userid":"accheung","firstName":"Null","usrRole":"Software Quality User","lastName":"Null","tiles":"Potential CFD",,"userType":"User"},
 {"objId":"5d9d7ce6fe506f11b275d01b","userid":"accheung","firstName":"Null","usrRole":"Software Quality User","lastName":"Null","tiles":"Potential CFD","userType":"User"}]

def function_apply(a):
    for d in a:
        if 'userid' in d:
            d['UserId'] = d.pop('userid')
        if 'userType' in d:
            d['User Type'] = d.pop('userType')
        if 'usrRole' in d:
            d['Role'] = d.pop('usrRole')
        if 'tiles' in d:
            d['Tiles'] = d.pop('tiles')
    return a

Now I want to convert same above function in lambda using map, but getting error. How could I achieve this?

Please add you attempt script of your lambda function. Thanks — EnriqueBet
– EnriqueBet, Commented Apr 6, 2020 at 15:12
Why do you assume a lambda function will be faster than a named function? — user2390182
– user2390182, Commented Apr 6, 2020 at 15:13
Lambda expressions produce exactly the same type (function) of values as def statements. — chepner
– chepner, Commented Apr 6, 2020 at 15:13
Also, you should not use comprehensions or map for mutational side effects. — user2390182
– user2390182, Commented Apr 6, 2020 at 15:15
In any case, you are iterating over the dicts in a list and modifying them in-place. A for loop is exactly the right construct to be using here, not map. — chepner
– chepner, Commented Apr 6, 2020 at 15:15

arnaud · Accepted Answer · 2020-04-06 15:38:49Z

4

A one-liner, concise, pythonic way:

mapping = {"userid": "UserId", "userType": "User Type", "usrRole": "Role", "tiles": "Tiles"}  
def rename(x): return [{mapping.get(k, k):v for (k,v) in d.items()} for d in x]

Please note that the usage of lambda is not providing any computation time gain. Regarding readability, a standard def is the most expected call for a function definition.

>>> rename(a)

[{'objId': '5c077187fe506f8dd3589ce6',
  'UserId': 'absurana',
  'firstName': 'Null',
  'Role': 'Software Quality User',
  'lastName': 'Null',
  'Tiles': 'Potential CFD',
  'User Type': 'User'},
 {'objId': '5d9d7ce6fe506f11b275d01b',
  'UserId': 'accheung',
  'firstName': 'Null',
  'Role': 'Software Quality User',
  'lastName': 'Null',
  'Tiles': 'Potential CFD',
  'User Type': 'User'},
 {'objId': '5d9d7ce6fe506f11b275d01b',
  'UserId': 'accheung',
  'firstName': 'Null',
  'Role': 'Software Quality User',
  'lastName': 'Null',
  'Tiles': 'Potential CFD',
  'User Type': 'User'}]

edited Apr 6, 2020 at 15:38

answered Apr 6, 2020 at 15:21

arnaud

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3 Comments

chepner Over a year ago

There is absolutely no reason to use a lambda expression here. They are intended for anonymous functions, not a way to make your code less readable.

arnaud Over a year ago

That's a very fair point, I actually (wrongly) use lambdas for one-liner short function definitions. But def also works, and would be more readable. Changing that.

chepner Over a year ago

Also, this creates a new dict in each case instead of modifying the original dict in-place. That may or may not be desirable.

chepner · Accepted Answer · 2020-04-06 15:33:21Z

2

What you have now is about as efficient as you can get. If you want to make the code less repetitive, you can do something like

key_map = {
    'userid': 'UserId',
    'userType': 'User Type',
    'usrRole': 'Role',
    'tiles': 'Tiles',
}

def function_apply(a):
    for old, new in key_map.items():
        for d in a:
            if old in d:
                d[new] = d.pop(old)

edited Apr 6, 2020 at 15:33

answered Apr 6, 2020 at 15:18

chepner

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2 Comments

MisterMiyagi Over a year ago

Minor quibble: move the key_map outside of the function so that it is created only once.

chepner Over a year ago

Good point; I was thinking CPython, at least, would create the dict when the function was defined, but it does not appear to do so. (Not that such a small dict takes much time to build, but still.)

user2390182 · Accepted Answer · 2020-04-06 15:30:55Z

You need a function that can be applied to the individual dicts, e.g:

keys = [
    ('userid', 'UserId'), ('userType', 'User Type'),
    ('usrRole', 'Role'), ('tiles', 'Tiles'),
]

def rename_keys(dct):
    for old, new in keys:
        if old in dct:
            dct[new] = dct.pop(old)

This function can now be applied to the list elements in a loop:

for d in a:
    rename_keys(d)

Using map to do this, is rather pointless:

map(rename_keys, a)

itself does nothing, as the changes will only become effective once the iterator is consumed. So you would have to do something like:

list(map(rename_keys, a))
# or
for _ in map(rename_keys, a): pass

Which illustrates why you shouldn't do it: one creates a spurious in-memory list, the other ends up with a loop anyway. If you don't use the return values of the function, don't use it in map or a comprehension.

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