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== 6.3#h(0.6em)Variable scope


  
    
=== Learning objectives


    By the end of this section you should be able to


    - Identify the scope of a program's variables.
- Discuss the impact of a variable's scope.


  
  
    
=== Global scope


    A variable's #strong[scope] is the part of a program where the variable can be accessed. A variable created outside of a function has #strong[global scope] and can be accessed anywhere in the program. A Python program begins in global scope, and the global scope lasts for the entire program execution.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Global variables in a program with a function]


      #link("https://www.openstax.org/r/global-variables")[Global variables in a program with a function; ch 6, video 6]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Global variables]
]


  
  
    
=== Local scope


    A variable created within a function has #strong[local scope] and only exists within the function. A local variable cannot be accessed outside of the function in which the variable was created. After a function finishes executing, the function's local variables no longer exist.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Global and local variables in a program with a function]


      #link("https://www.openstax.org/r/global-and-local-variables")[Global and local variables in a program with a function; ch 6, video 7]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Local variables]
]


  
  
    
=== Using local and global variables together


    Python allows global and local variables to have the same name, which can lead to unexpected program behavior. A function treats a variable edited within the function as a local variable unless told otherwise. To edit a global variable inside a function, the variable must be declared with the global keyword.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Editing global variables in a program with a function]


      #link("https://www.openstax.org/r/editing-global-variables")[Editing global variables in a program with a function; ch 6, video 8]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Using both local and global variables]


      Consider the following variations on the example program with the input 9.
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Benefits of limiting scope]


      A programmer might ask, "Why not just make all variables global variables to avoid access errors?" Making every variable global can make a program messy. Ex: A programmer debugging a large program discovers a variable has the wrong value. If the whole program can modify the variable, then the bug could be anywhere in the large program. Limiting a variable's scope to only what's necessary and restricting global variable use make a program easier to debug, maintain, and update.
]


  
  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Battle royale game launch]


    Write a program that reads in a selected game mode and calls one of two functions to launch the game. If the input is "br", call battle\_royale(). Otherwise, call practice().


    battle\_royale():


    - Reads in the number of players.
- Computes the number of teammates still needed. A full team is 3 players.
- Calls the function find\_teammates() with the calculated number.
- Prints "Match starting . . .".


    practice():


    - Reads in a string representing the desired map.
- Prints "Launching practice on \[desired map\]".


    Note: find\_teammates() is provided and does not need to be edited.


    Given input:


    
    br
    1
    The output is:


    
    Finding 2 players...
    Match starting...
    Given input:


    
    p
    Queen's Canyon
    The output is:


    
    Launching practice on Queen's Canyon
    def find\_teammates(num):
    print("Finding", num, "players...")

\# Define battle\_royale() and practice()

game\_mode = input()
\# Launch selected game mode
]