Python, Why?

Question

Why is not the output :
[[1,1,1,1], [1,1,1,2],...., [2, 2, 2, 1], [2, 2, 2, 2]]
, what's happen?!

https://code.sololearn.com/cZdM9X1R6ZYD/?ref=app

Answer 1

The answer is simple

a = [1]
b = a
a.append(2)

Now b == [1, 2]
Because we said that b=a

The same thing is happening when passing the argument l recursively and appending it to list, every time you update l, every l in list gets "updated"

Answer 2

because you are using mutable type 'list' as default arguments..

"When Python executes a “def” statement, it takes some ready-made pieces (including the compiled code for the function body and the current namespace), and creates a new function object. When it does this, it also evaluates the default values."
source :- https://web.archive.org/web/20200221224620/http://effbot.org/zone/default-values.htm

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SOLUTION :
use mutable types as default argument like this -->

def func( lst=None ):
if lst is None :
lst=[]
#your code goes here

https://code.sololearn.com/cCDNe6i5vNFh/?ref=app