python - Why the following is allowed in C? -

why following behavior allowed implementations of c ? if use variable define it, this:

#include <stdio.h>  int main() {     int = 9;     int b = ( +b );     printf("%d",b); } 

i know garbage value of b used there should compile time warnings . sounds little weird me. in languages python illegal such stuff :

>>> b = 9 >>> = + b  traceback (most recent call last):   file "<pyshell#2>", line 1, in <module>     = + b nameerror: name 'a' not defined 

i guess python not put variable current namespace until well-defined (i.e. has value within current scope) while in c above statement breaks this:

int b = 9; int a; // put namespace or whatever lookup table garbage value = + b; 

it nice if points out philosophy behind this.

explanation

in c function, of local variable declarations happen straight away. essentially, compiler translates code into:

int a; int b; // start of function = 9; b = + b; // end of function 

so can see, there variable b, exists whole runtime of function.

in python, doesn't happen. b not exist until assigned first time.

the python interpreter runs code a+b before runs b = <the result of a+b>. if think it's doing intermediate result equivalent to:

a = 9; intermediate_value = + b b = intermediate_value 

it clear using b before defined.

philosophy

this comes down difference between static language c, , dynamic language python. in dynamic language, lot of things done compile time in static language done @ runtime instead. such things can include modifying code of running program itself. (in theory, unfettered access computer's memory, in c too; on modern operating systems, impossible).

have read of page http://en.wikipedia.org/wiki/dynamic_programming_language more information.