It is natural to see functions within expressions, even as arguments to another function. The expression abs(5-len("counterparty")) shows uses two functions, abs and len. To understand how Python evaluates such expressions, the example below traces through some functions by putting print statements into the function (even though the print statements serve no useful purpose, they have the benefit of forcing output so that we can see the order of evaluation).

Functions of Functions

def double(x):
  print "by",
  return 2*x
def increment(y):
  print "one",
  return 1+y
def incdouble(z):
  r = increment(double(increment(z)))
  print "results are"
  return r
v = incdouble(5)
print "calculated to be =", v