Python lambda gotcha

lambda.py

def callback(message):
  print(message)

foo = ["a", "b", "c"]

# Environment variables are only captured as references by the lambda:
bar = [lambda: callback(s) for s in foo]
for f in bar:
  f() # => c x 3

# Arguments are saved in the lambda's scope:
bar = [lambda t = s: callback(t) for s in foo]
for f in bar:
  f() # => a, b, c

# It also will work if you switch to a generator, because the following three things happen sequentially:
#   * Next value of foo is found
#   * Lambda is created
#   * Lambda is evaluated (while s still contains the expected value).
bar = (lambda: callback(s) for s in foo)
for f in bar:
  f() # => a, b, c