stdlib_examples.md

Situations where increments may be useful

This list shows situations where increments (if used with care) may allow to avoid repetitions or make code more readable. Each situation is demonstrated by an example of the code from the Python's standard library to ensure that this situation is realistic (this approach is inspired by the "Importance of real code" section of PEP 572 "Assignment Expressions"). Only one example is shown for clarity, however the standard library typically contains many similar examples.

By providing this list, I do not claim that using increments and/or allowing them makes Python code better in general. See the description of possible risks here.

1. if conditions

• Source: multiprocessing/managers.py

  Current:

  self.id_to_refcount[ident] -= 1
  if self.id_to_refcount[ident] == 0:
      del self.id_to_refcount[ident]

  Improved:

  if --self.id_to_refcount[ident] == 0:
      del self.id_to_refcount[ident]

  This allows to avoid repeating the long subscription expression.

2. while conditions

• Source: unittest/util.py

  Current:

  e = expected[i]
  a = actual[j]
  if e < a:
      missing.append(e)
      i += 1
      while expected[i] == e:
          i += 1
  elif e > a:
      unexpected.append(a)
      j += 1
      while actual[j] == a:
          j += 1
  else:
      i += 1
      try:
          while expected[i] == e:
              i += 1
      finally:
          j += 1
          while actual[j] == a:
              j += 1

  Improved:

  e = expected[i]
  a = actual[j]
  if e < a:
      missing.append(e)
      while expected[++i] == e:
          pass
  elif e > a:
      unexpected.append(a)
      while actual[++j] == a:
          pass
  else:
      try:
          while expected[++i] == e:
              pass
      finally:
          while actual[++j] == a:
              pass

  This allows to make code much shorter.

3. Incrementing callbacks

• Source: test/test_weakref.py

  Current:

  def callback(w):
      self.cbcalled += 1
  
  o = C()
  r1 = MyRef(o, callback)
  r1.o = o
  del o

  Improved:

  o = C()
  r1 = MyRef(o, lambda: ++self.cbcalled)
  r1.o = o
  del o

  This allows to avoid the full function definition in a separate place.

4. Handmade enums

• Source: distutils/log.py

  Current:

  DEBUG = 1
  INFO = 2
  WARN = 3
  ERROR = 4
  FATAL = 5

  Improved:

  _index = 0
  DEBUG = ++_index
  INFO = ++_index
  WARN = ++_index
  ERROR = ++_index
  FATAL = ++_index

  This makes adding new values and reordering the existing ones easier.