Advanced topics

Writing Functions in Python

Shayne Miel

Software Architect @ Duo Security

Nested contexts

def copy(src, dst):
  """Copy the contents of one file to another.

  Args:
    src (str): File name of the file to be copied.
    dst (str): Where to write the new file.
  """

  # Open the source file and read in the contents
  with open(src) as f_src:
    contents = f_src.read()

  # Open the destination file and write out the contents
  with open(dst, 'w') as f_dst:
    f_dst.write(contents)

Nested contexts

with open('my_file.txt') as my_file:
  for line in my_file:
    # do something

Nested contexts

def copy(src, dst):
  """Copy the contents of one file to another.

  Args:
    src (str): File name of the file to be copied.
    dst (str): Where to write the new file.
  """

  # Open both files
  with open(src) as f_src:
    with open(dst, 'w') as f_dst:

      # Read and write each line, one at a time
      for line in f_src:
        f_dst.write(line)

Handling errors

def get_printer(ip):
  p = connect_to_printer(ip)

  yield

  # This MUST be called or no one else will
  # be able to connect to the printer
  p.disconnect()
  print('disconnected from printer')


doc = {'text': 'This is my text.'}

with get_printer('10.0.34.111') as printer:
  printer.print_page(doc['txt'])

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    printer.print_page(doc['txt'])
KeyError: 'txt'

Handling errors

try:
  # code that might raise an error
except:
  # do something about the error

finally:
  # this code runs no matter what

Handling errors

def get_printer(ip):
  p = connect_to_printer(ip)

  try:
    yield
  finally:
    p.disconnect()
    print('disconnected from printer')


doc = {'text': 'This is my text.'}

with get_printer('10.0.34.111') as printer:
  printer.print_page(doc['txt'])

disconnected from printer
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    printer.print_page(doc['txt'])
KeyError: 'txt'

Context manager patterns


Open	Close
Lock	Release
Change	Reset
Enter	Exit
Start	Stop
Setup	Teardown
Connect	Disconnect

¹ Adapted from Dave Brondsema's talk at PyCon 2012: https://youtu.be/cSbD5SKwak0?t=795

Let's practice!

Writing Functions in Python