Why are global variables evil?

python global-variables side-effects
LarsVegas picture LarsVegas · Oct 3, 2013 · Viewed 73.2k times · Source

I'm trying to find out why the use of global is considered to be bad practice in python (and in programming in general). Can somebody explain? Links with more info would also be appreciated.

Answer

Erik Kaplun picture Erik Kaplun · Oct 3, 2013

This has nothing to do with Python; global variables are bad in any programming language.

However, global constants are not conceptually the same as global variables; global constants are perfectly harmless. In Python the distinction between the two is purely by convention: CONSTANTS_ARE_CAPITALIZED and globals_are_not.

The reason global variables are bad is that they enable functions to have hidden (non-obvious, surprising, hard to detect, hard to diagnose) side effects, leading to an increase in complexity, potentially leading to Spaghetti code.

However, sane use of global state is acceptable (as is local state and mutability) even in functional programming, either for algorithm optimization, reduced complexity, caching and memoization, or the practicality of porting structures originating in a predominantly imperative codebase.

All in all, your question can be answered in many ways, so your best bet is to just google "why are global variables bad". Some examples:

If you want to go deeper and find out why side effects are all about, and many other enlightening things, you should learn Functional Programming:

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