1. Intro to CS Nov 29, 2016

2. Today
Functions
Understanding scope
Arguments
Return Value
Generator

3. Function Definition def function-name(): statements-to-be-executed
Function-name can not be a keyword or a built-in function

4. Functions
Program flow resumes at the point directly following the function call
Functions provide modularity and can be called repeatedly

5. Scope
Variables created outside functions can be referenced by statements inside functions. They have global scope
Variables created inside functions can not be referenced from outside the function. They have local scope.

6. Local Variables
Local variables of the same name can appear in different functions without conflict
You can coerce a local variable to make it global with the “global” keyword followed by the variable name only
Where a global variable and a local variable have the same name, the function uses the local variable.

7. Scope of Variables global_var = 1 def my_vars():
print('Global variable:', global_var)
local_var = 2
print('Local variable:', local_var)
global inner_var
inner_var = 3
my_vars()
print('Coerced global:', inner_var)

8. Arguments
An “argument” name can be specified between the parentheses in the function definition
def echo (user):
print(‘User:’, user)
A value can be specified in the parentheses when the function is called
echo(‘Mike’)
The function can reference the “passed in” value via the argument name

9. Multiple Arguments
Multiple arguments can specified by a comma-separated list
The call to the function must include the same number of values as arguments
The passed values must appear in the same order as the arguments
Unless the arguments are specified

10. Multiple Arguments def echo(user, lang, sys):
print('User:', user, 'Language:', lang, 'Platform:', sys)
echo('Mike', 'Python', 'Windows')
echo(lang='Python', sys='Mac OS', user='Anne')

11. Default Arguments Default value can be specified in the argument
Used by the function when no value gets passed by the caller
Default value gets overridden when the caller specifies a value for that argument

12. Default Arguments def mirror(user = 'Carole', lang = 'Python'):
print('\nUser:', user, 'Language:', lang)
mirror()
mirror(lang = 'Java')
mirror(user = 'Tony')
mirror('Susan', 'C++')

13. Return Value
A function can return a value to the caller using the “return” keyword
The returned result may be assigned to a variable by the caller
The returned result can be used directly in-line such as in a print statement

14. Return Value num = input('Enter an integer:') def square(num):
if not num.isdigit():
return 'Invalid Entry'
num = int(num)
return num * num
print(num, 'Squared is:', square(num))

15. Generator
A generator is a special function that returns a “generator object” rather than a data value
This retains the state of the function when it was last called
It will continue from the point when next called

16. Yield Generator function contains the ‘yield’ keyword
Specifies the generator object to be returned to the caller
When the ‘yield’ statement gets executed, the state of the generator object is frozen and the current value is retained

17. Next
The generator object returned by the yield statement can be assigned to a variable
The built-in next() function can take the generator object to continue execution of the function from the point it was frozen

18. Generator example def incrementer(): i = 1 while True: yield i i += 1
incr = incrementer()
print(next(incr))

19. Summary
Functions
Understanding scope
Arguments
Return Value
Generator