there must be some difference between python lambda and regular function?

Question

I tried 2 snippets to figure out the difference between python lambda and regular function.

this one get what is expected.

b = range(6)
def greater_than2(b):
    if b > 2:
        return b

print(list(filter(lambda x: x > 2, b)))
print(list(filter(greater_than2, b)))

both print give [3, 4, 5].

but something goes with this one

b = range(6)

def less_than2(b):
    if b < 2:
        return b

print(list(filter(lambda x: x < 2, b)))
print(list(filter(less_than2, b)))

i got following output

[0, 1]
[1]

so, what the difference between the lambda and less_than2 function?

Your functions should be returning true or false, not b. When b is zero, less_than2(b) returns 0, which is a falsey value. — khelwood
– khelwood, Commented Oct 30, 2018 at 14:41

jpp · Accepted Answer · 2018-10-30 14:46:23Z

3

Your functions are not comparable, greater_than2 needs to return a Boolean:

def greater_than2(b):
    return b > 2

The function defined above will return True when b > 2, or False when b <= 2.

Your regular functions return b. Let's consider what happens with your second example, where you see a discrepancy:

b = 0: less_than2 returns 0
b = 1: less_than2 returns 1
b = 2: less_than2 returns None
...
b = 5: less_than2 returns None

Notice if your if condition is not satisfied, a return statement is never met and therefore your function will return None. The only "Truthy" value of these return values is 1 as bool(0) and bool(None) evaluate to False.

edited Oct 30, 2018 at 14:46

answered Oct 30, 2018 at 14:41

jpp

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