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pythonfor-loopscope
Kamen Hristov
Kamen Hristov

Reputation: 21

For loop overriding outer variables instead of creating new ones

I stumbled upon another basic concept I've missed in Python:

Having this basic for (foreach) loop:

x = 15
for x in range(10):
    continue
print(x)

The value for x I expected was 15, but instead I got 9.

The same code snippet in C returns x's original value – 15:

#include <stdio.h>

int main(void) 
{
  int x = 15;
  for (int x = 0; x < 10; x++)
  {
    continue;
  }
  printf("%d", x);
  return 0;
}

I can't figure out how the variable scope works here.

Since x is declared outside the for loop scope, shouldn't a new local variable be created during the lifetime of the loop?

Why is x being overridden in the Python version then?

Upvotes: 1

Views: 366

Answers (1)

Matthieu Brucher
Matthieu Brucher

Reputation: 22033

This is not the same. In C, you explicitly create a new variable, whereas in Python, you reuse the name in the for scope, ending up overriding the previous value.

So the C equivalent really is:

#include <stdio.h>

int main(void) 
{
  int x = 15;
  for (x = 0; x < 10; ++x)
  {
    continue;
  }
  --x; // To accommodate the different behavior of the range loop
  printf("%d", x);
  return 0;
}

Don't forget that in Python, variables are just entries in a dictionary, dynamically created, whereas in C, they are independent, static items.

Upvotes: 3

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