Download Understanding Functions in Python: Definition, Call, Parameters, and Return Statements and more Schemes and Mind Maps Anatomy in PDF only on Docsity!
We Write Programs to Do Things
ā¢ Functions are the key doers
Function Call Function Definition
- Command to do the function
>>> plus( 23 )
- Defines what function does
def plus(n):
return n+
- Parameter : variable that is listed within
the parentheses of a method header.
- Argument : a value to assign to the method
parameter when it is called
Function
Header Function
Body
(indented)
Anatomy of a Function Definition
def plus(n):
"""Returns the number n+
Parameter n: number to add to
Precondition: n is a number"""
x = n+
return x
Function Header
name parameters
Docstring
Specification
Statements to
execute when called
The vertical line
indicates indentation
Use vertical lines when you write Python
on exams so we can see indentation
The return Statement
ā¢ Format : return < expression>
Ā§ Used to evaluate function call (as an expression)
Ā§ Also stops executing the function!
Ā§ Any statements after a return are ignored
ā¢ Example : temperature converter function
def to_centigrade(x):
"""Returns: x converted to centigrade"""
return 5*(x-32)/9.
A More Complex Example
Function Definition
def foo(a,b):
"""Return something
Param a: number
Param b: number"""
x = a
y = b
return x*y+y
Function Call
>>> x = 2
>>> foo(3,4)
x?
What is in the box?
A: 2
B: 3
C: 16
D: Nothing!
E: I do not know
- Number of statement in the
function body to execute next
Draw parameters
as variables
(named boxes)
Understanding How Functions Work
ā¢ Function Frame : Representation of function call
ā¢ A conceptual model of Python
function name
local variables (later in lecture)
parameters
instruction counter
Text (Section 3.10) vs. Class
Textbook This Class
def to_centigrade(x):
return 5*(x-32)/9.
Definition : Call : to_centigrade(50.0)
to_centigrade 1
x 50.
to_centigrade
x ā > 50.
Example: to_centigrade(50.0)
- Draw a frame for the call
- Assign the argument value
to the parameter (in frame)
- Execute the function body
Ā§ Look for variables in the frame
Ā§ If not there, look for global
variables with that name
- Erase the frame for the call
def to_centigrade(x):
return 5*(x-32)/9.
to_centigrade 1
x
Initial call frame
(before exec body)
next line to execute
Example: to_centigrade(50.0)
- Draw a frame for the call
- Assign the argument value
to the parameter (in frame)
- Execute the function body
Ā§ Look for variables in the frame
Ā§ If not there, look for global
variables with that name
- Erase the frame for the call
def to_centigrade(x):
return 5*(x-32)/9.
to_centigrade
x
Executing the
return statement
Return statement creates a
1 special variable for result
RETURN
Call Frames vs. Global Variables
The specification is a lie :
def swap(a,b):
"""Swap global a & b"""
tmp = a
a = b
b = tmp
>>> a = 1
>>> b = 2
>>> swap(a,b)
a 1 b 2
swap
a 1 b 2
Global Variables
Call Frame
tmp 1
x x
Global Space
(for globals.py)
Function Access to Global Space
ā¢ All function definitions
are in some module
ā¢ Call can access global
space for that module
Ā§ math.cos: global for math
Ā§ temperature.to_centigrade
uses global for temperature
ā¢ But cannot change values
Ā§ Assignment to a global
makes a new local variable!
Ā§ Why we limit to constants
get_a 1
a 4
globals.py
"""Show how globals work"""
a = 4 # global space
def get_a():
return a # returns global
Global Space
(for globals.py)
Function Access to Global Space
ā¢ All function definitions
are in some module
ā¢ Call can access global
space for that module
Ā§ math.cos: global for math
Ā§ temperature.to_centigrade
uses global for temperature
ā¢ But cannot change values
Ā§ Assignment to a global
makes a new local variable!
Ā§ Why we limit to constants
change_a
a 3.
a 4
globals.py
"""Show how globals work"""
a = 4 # global space
def change_a():
a = 3.5 # local variable
return a
Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
Param y: a number"""
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
What does the
frame look like
at the start?
