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Scoping and Namespaces

Scoping and Namespaces. CSIS 1595: Fundamentals of Programming and P roblem Solving 1. Namespaces. Namespace: List of all variables and their current values Maintained by Python environment Example:. Namespaces and Functions. Each function has different namespace

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Scoping and Namespaces

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  1. Scoping and Namespaces CSIS 1595: Fundamentals of Programming and Problem Solving 1

  2. Namespaces • Namespace: • List of all variables and their current values • Maintained by Python environment • Example:

  3. Namespaces and Functions • Each function has different namespace • Variable in function has no effect on same variable in any other function • Like two “separate universes” Spock in main program Spock in separate function

  4. Namespace Example

  5. Arguments and Namespaces • Arguments to functions are also local to a namespace • Example: Fahrenheit to Celsius program • Note that f and c are differentin function, main program

  6. Arguments and Namespaces • Key idea: Does not matter whether same or different variable names used in functions • Will be treated as different variables regardless • Example: Two versions of fahreheit2celsius program • Both do the same thing

  7. Passing Arguments by Value • Key idea: • Variables not passed or returned • Only their values are passed/returned • In both cases the valueof 50 passed and the value of 10 returned

  8. Namespaces and Functions • Why is this important? • Same function may be called from multiple places with different arguments • Example: “Largest of 4 numbers” program • Programs often created by hundredsof programmers • Each creates set of functions • Should not have to worry about what variables other programmers are using! • Functions often put in separate files (modules) • No main program associated with it!

  9. Functions Calling Functions • Separate namespace created for each call

  10. Activation Stack • Each current namespace = “activation record” • Maintained on “stack” of activation records • When function called, its namespace pushed on top of stack • When return from function, that namespace removed • Return to function immediately below it on stack

  11. Stack for Population Program compute_population print_line compute_population print_line get_years print_table print_table print_table print_table print_table print_table print_table print_table Main program Main program Main program Main program Main program Main program Main program Main program Main program Main program Main program

  12. Scoping • Key idea: Namespace only exists while function active • Scope of variable = function it is defined in • Does not exist outside of its scope(error if referenced)

  13. Scoping and Global Variables • Exception: Functions may refer to variables in main • Exception to the exception: If value assignedin function it becomes local to function • Just avoid doing this! Prints 3 Still 3

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