Tuesday 31 March 2009

Python variable binding semantics

What do the six following chunks of code do, and what would you like them to do? Which do you prefer?
funcs = []
for x in (1, 2, 3):
    funcs.append(lambda: x)
for f in funcs:
    print f()

funcs = []
for x in (1, 2, 3):
    def myfunc():
        return x
    funcs.append(myfunc)
for f in funcs:
    print f()

funcs = []
for x in (1, 2, 3):
    funcs.append(lambda x=x: x)
for f in funcs:
    print f()

funcs = []
for x in (1, 2, 3):
    def g(x):
        funcs.append(lambda: x)
    g(x)
for f in funcs:
    print f()

funcs = []
def append_a_func(x):
    funcs.append(lambda: x)
for x in (1, 2, 3):
    append_a_func(x)
for f in funcs:
    print f()

def make_func(x):
    return lambda: x
funcs = []
for x in (1, 2, 3):
    funcs.append(make_func(x))
for f in funcs:
    print f()