What is the problem with shadowing names defined in outer scopes

Question

I just switched to PyCharm and I am very happy about all the warnings and hints it provides me to improve my code  Except for this one which I don t understand   This inspection detects shadowing names defined in outer scopes   I know it is bad practice to access variable from the outer scope  but what is the problem with shadowing the outer scope  Here is one example  where PyCharm gives me the warning message  data    4  5  6   def print data data      lt -- Warning   quot Shadows  data  from outer scope     print data  print data data

User · Accepted Answer

There isn t any big deal in your above snippet  but imagine a function with a few more arguments and quite a few more lines of code  Then you decide to rename your data argument as yadda  but miss one of the places it is used in the function s body    Now data refers to the global  and you start having weird behaviour - where you would have a much more obvious NameError if you didn t have a global name data  Also remember that in Python everything is an object  including modules  classes and functions   so there s no distinct namespaces for functions  modules or classes  Another scenario is that you import function foo at the top of your module  and use it somewhere in your function body  Then you add a new argument to your function and named it - bad luck - foo  Finally  built-in functions and types also live in the same namespace and can be shadowed the same way  None of this is much of a problem if you have short functions  good naming and a decent unit test coverage  but well  sometimes you have to maintain less than perfect code and being warned about such possible issues might help

User · Answer

Do this   data    4  5  6   def print data        global data     print data   print data

User · Answer

A good workaround in some cases may be to move the variables and code to another function  def print data data       print data  def main        data    4  5  6      print data data   main

User · Answer

The currently most up-voted and accepted answer and most answers here miss the point  It doesn t matter how long your function is  or how you name your variable descriptively  to hopefully minimize the chance of potential name collision   The fact that your function s local variable or its parameter happens to share a name in the global scope is completely irrelevant  And in fact  no matter how carefully you choose you local variable name  your function can never foresee  quot whether my cool name yadda will also be used as a global variable in future  quot   The solution  Simply don t worry about that  The correct mindset is to design your function to consume input from and only from its parameters in signature  That way you don t need to care what is  or will be  in global scope  and then shadowing becomes not an issue at all  In other words  the shadowing problem only matters when your function need to use the same name local variable and the global variable  But you should avoid such design in the first place  The OP s code does not really have such design problem  It is just that PyCharm is not smart enough and it gives out a warning just in case  So  just to make PyCharm happy  and also make our code clean  see this solution quoting from silyevsk s answer to remove the global variable completely  def print data data       print data  def main        data    4  5  6      print data data   main    This is the proper way to  quot solve quot  this problem  by fixing removing your global thing  not adjusting your current local function

User · Answer

It looks like it is 100  a pytest code pattern  See  pytest fixtures  explicit  modular  scalable I had the same problem with it  and this is why I found this post        tests test twitter1 py import os import pytest  from mylib import db         pytest fixture def twitter        twitter    db Twitter       twitter   debug   True     return twitter    pytest mark parametrize  quot query expected quot           quot BANCO PROVINCIAL quot   8         quot name quot   6         quot castlabs quot   42      def test search twitter  db Twitter  query  str  expected  int        for query in queries          res   twitter search query          print res          assert res  And it will warn with This inspection detects shadowing names defined in outer scopes  To fix that  just move your twitter fixture into   tests conftest py     tests conftest py import pytest  from syntropy import db    pytest fixture def twitter        twitter    db Twitter       twitter   debug   True     return twitter   And remove the twitter fixture  like in   tests test twitter2 py      tests test twitter2 py import os import pytest  from mylib import db         pytest mark parametrize  quot query expected quot           quot BANCO PROVINCIAL quot   8         quot name quot   6         quot castlabs quot   42      def test search twitter  db Twitter  query  str  expected  int        for query in queries          res   twitter search query          print res          assert res  This will be make happy for QA  PyCharm and everyone

User · Answer

data    4  5  6    Your global variable  def print data data      lt -- Pass in a parameter called  quot data quot      print data     lt -- Note  You can access global variable inside your function  BUT for now  which is which  the parameter or the global variable  Confused  huh   print data data

User · Answer

I like to see a green tick in the top right corner in PyCharm  I append the variable names with an underscore just to clear this warning so I can focus on the important warnings  data    4  5  6   def print data data        print data    print data data

User · Answer

It depends how long the function is  The longer the function  the greater the chance that someone modifying it in future will write data thinking that it means the global  In fact  it means the local  but because the function is so long  it s not obvious to them that there exists a local with that name  For your example function  I think that shadowing the global is not bad at all

[python] What is the problem with shadowing names defined in outer scopes?

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