Lesson 4 - Flow Control

Execute a code block if a condition is true

>>> x = 0
>>> if x == 0:
...     print("Zero")
...
Zero

Provide a fallback code block using else

>>> x = 1
>>> if x == 0:
...     print("Zero")
... else:
...     print("Non-zero number")
...
Non-zero number

Combine an else and if statement with elif

>>> x = 2
>>> if x == 0:
...     print("Zero")
... elif x % 2:
...     print("odd number")
... else:
...     print("Even number")
...
Even number

Non-zero numbers and non-empty iterables are considered True

These statements are equivalent for an iterable

if len(myIterable) > 0:

if len(myIterable):

if myIterable:

These statements are equivalent for a number

if myNumber != 0:

if myNumber

Return True if any elements in an iterable evaluate to True

>>> any([0, 1, 0])
True

Return True if all elements in an iterable evaluate to True

>>> all([True, 'string', [3, 4], 2])
True

Loop until condition is false

>>> x = 1
>>> while x <= 3:
...     print(x)
...     x += 1
...
1
2
3

break is used to end a loop

>>> x = 1
>>> while True:
...     print(x)
...     x += 1
...     if x > 3:
...             break
...
1
2
3

An else statement can be used to execute code whenever a while condition is false

Not executed when break is used

>>> x = 1
>>> while x <= 3:
...     print(x)
...     x += 1
... else:
...     print("I can only count to twee!")
...
1
2
3
I can only count to twee!

Loop over all values in an Iterable

>>> bears = ['Papa', 'Mama', 'Baby']
>>> for bear in bears:
...     print(bear)
...
Papa
Mama
Baby

continue is used to continue with a loop

>>> for num in [1, 2, 3, 4]:
...     if num % 2:
...             continue
...     print(num, "Even!")
...
2 Even!
4 Even!

break is used to end a loop before completion

>>> deadly = ['hippo', 'aligator', 'python']
>>> for animal in ['rabbit', 'horse', 'hippo', 'mouse']:
...     print('What a cute %s!' % animal)
...     if animal in deadly:
...             print('Oops!')
...             break
...
What a cute rabbit!
What a cute horse!
What a cute hippo!
Oops!

When a loop fails to iterate, else can be used to run fallback code

This happens when a list is empty

numbers = []
>>> for num in numbers:
...     print(num)
... else:
...     print("Where are the numbers?")
...
Where are the numbers?

The range() function can be used to iterate over integers

Format is range(start, stop[, step])

• Final integer is one less than stop

>>> for num in range(1, 3):
...     print(num)
...
1
2
>>> for num in range(2, 6, 2):
...     print(num)
...
2
4
>>> for num in range(2, 0, -1):
...     print(num)
...
2
1

The enumerate() function returns tuples with a count and items from an iterable

Count starts at 0 by default

Format is enumerate(iterable, start=0)

>>> for num, entry in enumerate(['A', 'B', 'C'], 1):
...     print(num, entry)
...
1 A
2 B
3 C

List comprehension allows creating lists from inline for loops

>>> numbers = [-2, -1, 0, 1, 2, 3, 4]
# Square all numbers in a list
... squares = [x**2 for x in numbers]
>>> squares
[4, 1, 0, 1, 4, 9, 16]

# Square even numbers in a list
... evenSquares = [x**2 for x in myList if not x % 2]
>>> evenSquares
[4, 0, 4, 16]

Can be used in other expressions

>>> sum([x**2 for x in numbers])
35

In Python 2.7 and later, set and dictionary comprehension are available

Set comprehension

>>> numbers = [-2, -1, 0, 1, 2, 3, 4]
>>> {x**2 for x in numbers}
{0, 1, 4, 9, 16}

• Notice the new set syntax introduced in Python 2.7

Dictionary comprehension

>>> {x : x**2 for x in numbers}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, -2: 4, -1: 1}

When an error occurs, an exception is raised

>>> int('A')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'A'

Exceptions include:

• The type of exception
• An error message
• A traceback showing what code was being executed

Exceptions are not always bad, sometimes they are expected behavior

Exceptions can be caught using a try-except statement

>>> try:
...     int('A')
... except ValueError:
...     print('That value is invalid, please provide an integer')
...
That value is invalid, please provide an integer

Tips

• Very general exceptions may catch something unexpected
• Put only the code that will raise an exception in the try block

Multiple exceptions can be listed in a tuple

try:
    os.listdir(someDirectory)
except (OSError, IOError):
    print("Error opening %s" % someDirectory)

Multiple except statements can be used to handle different conditions

try:
    ...
except IOError:
    ...
except RuntimeError:
    ...
except KeyError
    ...

An else block can be used to execute code when no exception is raised

def divide(num1, num2):
    try:
        result = num1 / num2
    except ZeroDivisionError:
        print("Nope, still can't divided by zero")
    else:
        print("Result is %d" % result)

>>> divide(2, 1)
Result is 2
>>> divide(2, 0)
Nope, still can't divided by zero

A finally block can be included to execute code under all conditions

finally blocks are executed after all other code

If an unhandled exception occurs, the finally block will be executed before it is raised

Useful for closing files, releasing locks, or other cleanup tasks

lock.acquire()

try:
    ...
except IOError:
    ...
finally:
    lock.release()

In Python 2.6+ the exception can be caught with the as keyword

>>> try:
...     1/0
... except ZeroDivisionError as e:
...     print('An error occured: %s' % e)
...
An error occured: division by zero

To write code compatible with older versions of Python, use sys.exc_info()

>>> try:
...     1/0
... except ZeroDivisionError:
...     e = sys.exc_info()[1]
...     print('An error occured: %s' % e)
...
An error occured: division by zero

Exceptions can be raised with the raise keyword

>>> raise RuntimeError("This parrot is no more! ")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: This parrot is no more!

Here are simple examples for creating custom exceptions

• Classes will be covered in another lesson

class CustomException(Exception):
    """
    Custom exception description
    """
    pass


class CustomExceptionChild(CustomException):
    """
    Child of Custom exception description
    """
    pass

Custom exceptions should be created hierarchically