Mutation:

Python allows us to change the values of variables
The following Python program is valid:
x = “a”
x = 100
x = 2 * x - 1

Local vs Global variables

Variables defined inside a function are called local variables
– Local variables only can be mutated inside the function they are defined in

Variables defined outside a function are called global variables
– Global variables cannot be mutated inside any functions in CS116.

###Correct Usage of Global Variables: tax_rate = 0.13
def total_owed(amount):
return amount * (1+tax_rate)

This Causes an Error:

grade = 87
def increase_grade(inc):
      grade = grade + inc increase_grade(5)

Mutating Parameters

Consider the program:

def add1(n):
      n = n + 1
return n

starter = 0
y = add1(starter)

The value of n is changed locally, but the value of starter is not changed.The mutation of n is a local mutation only.
Even if starter was called n, the same behaviour would be observed.

Tip: Python expects each line of code to be an entire statement
Can be a problem e.g. due to indentation
If a statement is not done, use a \ (backslash) character to show it continues on next line

Built-in type Bool:

• True, False
• Equality testing: ==
• Use for all atomic values (except for floats)
• Inequality testing:<,<=,>,>=
• != is shorthand for not equal

Very similar to Scheme

v1 and v2
True only if both v1, v2 are True
v1 or v2
False only if both v1, v2 are False
not v
True if v is False , otherwise False

Like Scheme, Python uses Short-Circuit evaluation
Stop evaluating as soon as answer is known
or: stop when one argument evaluates to True
and: stop when one argument evaluates to False

Example of an If Statement

if test: true_action_1 … true_action_K

def double_positive(x):
      result = x
      if x > 0:
            result = 2*x
      return result