Control flow statements

You can set a conditional construct with if / elif / else keywords:

number = 10

if number < 20:
    print('less than twenty')
elif 20 <= number < 40:
    print('between twenty and forty')
else:   # number >= 40
    print('more than forty')

Python supports two flavours of loops, for and while loops. For loops are designed to browse sequences (or iterables). They provide an identifier pointing to the element made available at each cycle, eachChr in this case

for eachChr in 'some string':
    print(eachChr)

While loops instead are an extension of a simple conditional construct. They keep cycling until a certain condition is evaluated as True

from random import random

switch = True
while switch:
    print('looping...')

    if random() < 0.2:
        switch = False

The ‘try’ statement

# setup
try:
    # action
finally:
    # cleanup
  • finally is always executed, even if the code that does the work doesn’t finish

The ‘with’ statement

The with statement offers a convenient syntax to express the common pattern of setting up a context, perfoming an action, and then cleaning up when done.

A with statement works together with a context manager – an object that handles the entry into and exit from a context, encapsulating the setup and cleanup actions.

Here are a few examples of with in use, and the corresponding code without it:

Opening a file, processing its contents, and closing the file:

with open('example.txt', 'r') as f:
    txt = f.read()
# same as:
f = open('example.txt', 'r') # setup
try:
    txt = f.read() # action
finally:
    f.close() # cleanup

Managing the graphics state in DrawBot:

with savedState():
    scale(1.5)
# same as:
save() # setup
scale(1.5) # action
restore() # cleanup

Managing undo history in RoboFont:

with glyph.undo():
    glyph.removeOverlap()
# same as:
glyph.prepareUndo() # setup
glyph.removeOverlap() # action
glyph.performUndo() # cleanup
Last edited on 15/07/2021