Programming Patterns

This page contains standard 'patterns' that you should get used to. When you need to perform one of these tasks, follow the standard pattern.

Menus

Use the if/elif.../else pattern in Python (switch statements in other languages)

display menu
get choice
while choice != <quit option>
    if choice == <first option>
        <do first task>
    else if choice == <second option>
        <do second task>
    ...
    else if choice == <n-th option>
        <do n-th task>
    else
        display invalid input error message
    display menu
    get choice
<do final thing, if needed>

Error checking

<priming read - do something the loop depends on>
while <condition based on something from above>:
    display error message
    <same as the priming read again>
do next thing now that you know the 'something' is valid

Example:

age = int(input("Age: "))
while age < 0:
    print("Invalid age!")
    age = int(input("Age: "))
print("You are {} years old".format(age))

Exception-based error checking

Example:

valid_input = False
while not valid_input:
    try:
        age = int(input("Age: "))
        if age < 0:
            print("Age must be >= 0")
        else:
            valid_input = True
    except ValueError:  # or just  except:
        print("Invalid (not an integer)")
print("Next year you will be", age + 1)

Finding

for each item in items
    if item matches what we're looking for
        return result or set value and break

(note no need for else or continue, it will move to the next item)

Filtering

filtered_items
for each item in items
    if item matches what we want
        add item to filtered_items

(note no need for else or continue, it will move to the next item)

Note that Python (and many languages) have neat shortcuts for filtering, e.g. using list comprehensions:

filtered_items = [item for item in items if item matches what we want]

"Innocent until proven guilty" Boolean validating

...

Working with Booleans

You never need to compare to True or False. E.g. instead of:

if condition == True:
... or
if condition == False:

You can just use:

if condition:
... or
if not condition:

If you are ever returning (or setting a variable to) True/False depending on a condition, you can just return (or set) to the condition. So, instead of:

if condition:
    return True
else:
    return False

You can just use:

return condition