Caching in default arguments

badcache.py

# See
# https://stackoverflow.com/a/279592
# https://stackoverflow.com/a/9158327

from __future__ import print_function

def test(m, n, reg=0.0,
         _prm=[None, None], _M=[None, None],
         _reg=[None], _LR=[None, None]):

  print('m = {}, n = {}, reg = {}'.format(m, n, reg))
  
  if _prm != [m, n]:
    print('Generating M...')
    M1 = [i for i in range(m)]
    M2 = [n - i for i in range(n)]
    _prm[:] = [m, n]
    _M[:] = [M1, M2]
    _reg[:] = [None]
  else:
    print('Using M')
    M1, M2 = _M

  if _reg != [reg]:
    print('Updating LR...')
    L = [reg * x for x in M1]
    R = [reg * x for x in M2]
    _reg[:] = [reg]
    _LR[:] = [L, R]
  else:
    print('Using LR')
    L, R = _LR
  
  print('M1 =', M1)
  print('M2 =', M2)
  print('L =', L)
  print('R =', R)
  print()

test(10, 8)

test(10, 8)

test(10, 8, 0.5)

test(10, 8, 0.5)

test(8, 8, 0.5)

output

m = 10, n = 8, reg = 0.0
Generating M...
Updating LR...
M1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
M2 = [8, 7, 6, 5, 4, 3, 2, 1]
L = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
R = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

m = 10, n = 8, reg = 0.0
Using M
Using LR
M1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
M2 = [8, 7, 6, 5, 4, 3, 2, 1]
L = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
R = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

m = 10, n = 8, reg = 0.5
Using M
Updating LR...
M1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
M2 = [8, 7, 6, 5, 4, 3, 2, 1]
L = [0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5]
R = [4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5]

m = 10, n = 8, reg = 0.5
Using M
Using LR
M1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
M2 = [8, 7, 6, 5, 4, 3, 2, 1]
L = [0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5]
R = [4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5]

m = 8, n = 8, reg = 0.5
Generating M...
Updating LR...
M1 = [0, 1, 2, 3, 4, 5, 6, 7]
M2 = [8, 7, 6, 5, 4, 3, 2, 1]
L = [0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5]
R = [4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5]