conversion of a variable in python

Question

Hello :)

So i've got a question who keeps bothering me :

you know how in Python we can easily convert a string in integer by doing :
>>>> var = new_type(type)

for instance, number= int("2") will return 2 as an integer.

BUT i've been trying a piece of code like this :

>>>>> demand=input(": ")
base=int(len(demand))

i=0
temp="0"
nb_temp=0

temp+=demand[i]
i+=1
nb_temp=int(temp) <<<<<<

So, technically, (the code isn't complete here), let's imagine the user input is "15+21"

The idea here is to stock "1", then "5" in temp, so the variable is equal to the string "15".

then i want to fill another variable with the integer 15.

SO i try to convert it by the code above : nb_temp=int(temp), since temp is worth "15", and nb_temp=int("15") DOES work.

SO i don't get why i can't make it work with my variable o.o

Little edit, simplification :

i'm trying to do

*******************
a=input(": ")
b=0
c="0"
while(b<=len(a)):
c+=a[b]
d=float(c)
print(d)
************

wich is suppose to give the same result as

*****************
a=input(": ")
b=float(a)
print(b)
****************

Those two codes are supposed to works just as well, except the first one is way better to use, but seems not to work --'

Answer 1

indeed, I wrote my second example too quickly, thanks for pointing it out :)

but my initial difficulty stays the same, I don't get my it doesn't work ;-(

Answer 2

In your first example b is never incremented, so the while loop will never end.
while(b<=len(a)):