Control flow ↩

Control flow statements

• choice
  • if / elif / else
• loops
  • for
  • while
• context
  • with
• exceptions
  • try / except / finally

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

• The ‘with’ statement
• PEP 343 - The ‘with’ statement
• Context Manager Types