Describe functions in Python

Python Functions :
  • A function is a block of organized, reusable code that is used to perform a single, related action.
  • Python gives us many built-in functions like print(), etc. but you can also create your own functions. These functions are called user-defined functions.
  • Parameter Passing :
    • We can pass arguments to a function either by value or by reference.
    • Call by Value :
      • Sometimes it's also called pass by value.
      • In call-by-value, the argument expression is evaluated, and the result of this evaluation is bound to the corresponding variable in the function. So, if the expression is a variable, a local copy of its value will be used, i.e. the variable in the caller's scope will be unchanged when the function returns.
    • Call by Reference :
      • It is also known as pass-by-reference.
      • In this a function gets an implicit reference to the argument, rather than a copy of its value.
      • As a consequence, the function can modify the argument, i.e. the value of the variable in the caller's scope can be changed.
      • The function can modify the argument, i.e. the value of the variable in the caller's scope can be changed.
      • The disadvantage of it is that variables can be "accidentally" changed in a function call. So special care has to be taken to "protect" the values, which shouldn't be changed. 
  • What is used in Python?
    • Python uses a mechanism, which is known as "Call-by-Object", sometimes also called "Call by Object Reference" or "Call by Sharing".
    • If you pass immutable arguments like integers, strings or tuples to a function, the passing acts like call-by-value. The object reference is passed to the function parameters. They can't be changed within the function, because they can't be changed at all, i.e. they are immutable.
    • It's different, if we pass mutable arguments. They are also passed by object reference, but they can be changed in place in the function. If we pass a list to a function, we have to consider two cases: Elements of a list can be changed in place, i.e. the list will be changed even in the caller's scope. If a new list is assigned to the name, the old list will not be affected, i.e. the list in the caller's scope will remain untouched.

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