Why does this equal 4

def inc(a,b=1):

return(a+b)

a=inc(1)

a=inc(a,a)

print(a)

#inc function takes two parameter a and b, where parameter b default value is set to 1
#so if parameter b is not specified during function call it takes its value as 1
def inc(a,b=1):
    #returns sum of a and b
    return(a+b)


#calling inc function with single parameter 1, so parameter a becomes 1
# and b is not specified so it becomes 1(its default value)
a=inc(1)        
#result is, value of a is 2(1+1) now
#calling inc function again with two parameters both same(a) whose values are 2
#since we are passing value 2 to parameter b this time,  it takes value as 2
a=inc(a,a)
#result is value of a is 4(2+2) this time
#print value of a it is 4.
print(a)

Step-by-step explanation

1) Above is your program with inc function, I have added comments to explain it better

2) basically it has inc function which takes two parameter a and b, where parameter b default value is set to 1, so if parameter b is not specified during function call it takes its value as 1

3) In first inc function call "a=inc(1) ", we have specified value for a and b value is not specified, so function takes value of a as 1 and value of b also as 1(its default value).

4) It adds up and return 2 which gets assigned to a again

5) in second function call "a=inc(a,a)", we have specified value for a and b as a which is 2, so this function adds up 2+2 and return 4

6) result 4 is again assign to a and printed.

7) so output is 4