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How to shoot yourself in the foot with Python's variables scope

Got fooled again by Pythonic crap...

def do_something_with(p):
    print(f"do_something_with({p})")


def fun():
    # loop on some data
    for i in [0, 1, 2]:
        hoge = f"hoge{i}"
        do_something_with(hoge)
    # do some other processing related to the data
    pass
    # loop again on the data
    for i in [0, 1, 2]:
        # forget this line: hoge = f"hoge{i}"
        do_something_with(hoge)


fun()

# Produces:
# do_something_with(hoge0)
# do_something_with(hoge1)
# do_something_with(hoge2)
# do_something_with(hoge2)
# do_something_with(hoge2)
# do_something_with(hoge2)

The code above is a simplified version of a real bug encountered at work. The script executes without error, PEP8 doesn't see anything problematic, and in the original script the bug was completely hidden, only found much later by pure chance.

As written in the comment, the bug is that I forgot to reassign the hoge variable in the second loop. In Python, variables scope is global at function level, so the hoge variable of the first loop was used instead, with the value it has at the end of the first loop. Another occasion for Python to prove itself as a bullshit language by not following principles on which every experience programmer agrees (1, 2, 3, 4, 5, ...)

Try to do the same thing in a serious programming language, C for example:

#include "stdio.h"

void do_something_with(int p) {
  printf("do_something_with(%d)\n", p);
}

void fun() {
  // loop on some data
  for(int i = 0; i < 3; ++i) {
    int hoge = i;
    do_something_with(hoge);
  }
  // do some other processing related to the data
  ;
  // loop again on the data
  for(int i = 0; i < 3; ++i) {
    // forget this line: int hoge = i;
    do_something_with(hoge);
  }
}

int main() {
  fun();
  return 0;
}

and the compiler will yell back at you and save you the shame and pain two months later when the bug will resurface.

main.c: In function ‘fun’:
main.c:18:23: error: ‘hoge’ undeclared (first use in this function)
   18 |     do_something_with(hoge);
      |                       ^~~~
main.c:18:23: note: each undeclared identifier is reported only once for each function it appears in

2022-07-21
in All, Pub talk,
113 views