Lambda Function to return different value using parameters

Question

I would like to use a lambda function to return values. This could be handled in a function, however would prefer the use of lambda for this exercise. In general, I want to have the lambda return either a date or 0 by passing a second parameter.

import datetime
none_func=lambda x: 0 if x is None else x
print(none_func(None))
print(none_func(7))
print(none_func(datetime.datetime(2020,1,1)))
### Here, update the lambda function to take an additional parameters, like returns a date if None as well
none_func=lambda x: datetime.datetime(1900,1,1) if x is None else x
print(none_func(None))
print(none_func(7))

Updated for clarification:

I would like to convert the coalesce function to a lambda.

def coalesce(x,return_schema='int'):
    if x is None and return_schema=='int': return 0
    elif x is None and return_schema=='date': return datetime.datetime(1900,1,1)
    else: return x

print(coalesce(7))
print(coalesce(None))
print(coalesce(None,return_schema='date'))
print(coalesce(datetime.datetime.today(),return_schema='date'))

I think your main question is how to write a lambda expression with optional parameters. Am I right? — SomethingSomething, Mar 12 '20 at 07:52
Surprisingly, it is possible: `>>> f = lambda a, b=0 : a + b ;>>> f(5) ;5 >>> f(5, 2) ;7 ` — SomethingSomething, Mar 12 '20 at 07:55
You have the technical answer below... but it's totally unpythonic - it's using a lambda for no good reason (lambdas are supposed to be _anonymous_ functions) and turns a simple readable code into a complex one-liner that requires twice much effort to read and understand. — bruno desthuilliers, Mar 12 '20 at 08:10
I agree with you. Later to realize its best just to use a defined function instead. For the most part, I was trying to understand how to use a lambda to replace it, however, as you pointed out a labda in this instance would make for a difficult read. — kdb, Mar 12 '20 at 09:17

Ángel Igualada · Accepted Answer · 2020-03-12T08:17:20.070

1

Use the function would be a better option, but if you want to convert it to lambda you can try something like this:

import datetime

f = lambda x,return_schema="int": 0  if x is None and return_schema=='int' else ( datetime.datetime(1900,1,1) if x is None and return_schema=='date' else x)
print(f(7))
print(f(None))
print(f(None, return_schema='date'))
print(f(datetime.datetime.today(), return_schema='date'))

I hope it helps.

edited Mar 12 '20 at 08:17

answered Mar 12 '20 at 07:56

Ángel Igualada

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I agree, the function would be a better option, but i think this is only to get practice in how you can build lambda expressions and try other ways. – Ángel Igualada Mar 12 '20 at 08:16
Yes that's what I understand too - but it's still usefull for python newbies to be warned that it's not what they should do ;-) – bruno desthuilliers Mar 12 '20 at 08:21
1

I added that in the response header, thanks for remark that important point. – Ángel Igualada Mar 12 '20 at 08:22

score 0 · Answer 2 · answered Mar 12 '20 at 07:49

Are you looking for something along the lines of:


func = lambda x, y: 'default' if (x is None or y is None) else x

func(1, 2)        # returns 1
func(None, 1)     # returns 'default'
func(1, None)     # returns 'default'
func(None, None)  # returns 'default'

Or if you only want to do something based on the second parameter just change the if statement to test the value of that.

Lambda Function to return different value using parameters

Updated for clarification:

2 Answers2