#🔒 can someone explain me why arg used in the code below

69 messages · Page 1 of 1 (latest)

lavish moon
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is_even_list = [lambda arg=x: arg * 10 for x in range(1, 5)]
for item in is_even_list:
print(item())

sullen brookBOT
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@lavish moon

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outer leaf
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what is the output you observed when erasing arg=x?

lavish moon
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wait a sec

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Traceback (most recent call last):
File "D:\pythonProject\first file.py", line 6, in <module>
print(item())
^^^^^^
TypeError: <lambda>() missing 1 required positional argument: 'x'

outer leaf
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no, not even with x remaining

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only like lambda: x * 10

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not lambda x: x * 10

lavish moon
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ok

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40
40
40
40

outer leaf
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yeah

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so the thing with lambdas is that

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the names used in its body are looked up lately

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meaning, it looks it up only at the time of execution, not definition

lavish moon
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ohh

outer leaf
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if we consider this:

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[lambda: x * 10 for x in range(1, 5)]
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this is a list of 4 lambdas alright

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but after this list is made, when you call any of them

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it will attempt x * 10 right?

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but what is x?!

lavish moon
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yess

outer leaf
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it didn't "register" it when defining, so it goes back to its definition scope at the execution time and sees what the x is

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and guess what

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x was last 4 there

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right?

lavish moon
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yess

outer leaf
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after the list is made, x was left off 4

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so all of them uses 4

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and we get 40 40 40 40

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makes sense?

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people came up with "solutions" to this

lavish moon
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then how can i get the desired output

outer leaf
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one way is making a "dummy" parameter like your original code had

lavish moon
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ok

outer leaf
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somewhat more confusingly, convention is like:

[lambda x=x: x * 10 for x in range(1, 5)]
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this forces early binding if you will

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because at the definition time, x must be evaluated

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but here there is 2 xs, which is confusing

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so your code is better in that sense:

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[lambda arg=x: arg * 10 for x in range(1, 5)]
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here, when the lambdas are being defined, their default values for parameters are defined at that time

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and x is each turn appointed to arg with 1, 2, 3 and 4

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this way you captured the definition-time x into arg

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so when you further call the lambdas with no argument like item() you did

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default values will be used

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and the default values were correctly registered at definition time with desired values of 1, 2, 3 and 4

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so we get 10 20 30 40 as expected

lavish moon
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can i connect with u personally

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?

outer leaf
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in a spiritual level? :p

lavish moon
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noop

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as a student ig

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i got lots of doubts

outer leaf
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i mean you can ask questions here i'm not always available as you expect

lavish moon
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or we can be friends

outer leaf
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sure

spare marten
outer leaf
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alluding to late binding

spare marten
spare marten
outer leaf
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i meant late-ly :p

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English strikes me again

spare marten
outer leaf
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:p i think i have at least threeteen now

spare marten
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onety three

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😎

sullen brookBOT
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