The code that follows is the result of noodling around with switches as a
learning tool. I've played with python for a few years, but I'm self-taught, so
. . .
Class Switch builds a set of functions. Method switch executes one of them
given a value of the switch variable.
My question is, why are modules imported at the top of the program not visible
to the functions Switch builds? There is
no problem when the import is in the function, but I thought initially that
imports at the top would be in its globals.
The import that works is at line 111 in the code.
Thanks in advance!
George
'''Mixin Switch provides function switch(key) that executes an appropriate
function.
Each instance can: use a different switch variable.
be used as many places in a program as desirable.
be changed in one place, no matte how many places it is
used.
Usage: inst = Switch(keys, vals, base)
whose arguments are sequenes:
keys has switch values ('su', 'mo', . . .),
base has the shared fore and aft parts of instance functions, and
vals has the individual parts of instane functions.
Example: Suppose you want to switch on days of the week:
keys = ('su', 'mo', 'tu', 'we', 'th', 'fr', 'sa', 'de')
vals = ('Sundayis Comic-day.', 'Mondayis Moan-day.',
'Tuesday is Twos-day.', 'Wednesday is Hump-day.',
'Thursday is Shop-day.', 'Fridayis TGIF-day.',
'Saturday is Food-day.', 'Anything else is Party-day!')
fore = "def %s(self, *args):\n\tprint '"
aft = "'\\n"
produces functions of the form:
def su(self, *args):\\n\\tprint 'Sundayis Comic-day.'\\n
or, for humans:
def su(self, *args):
print 'Sundayis Comic-day.'
Test code (below) for this example produces:
Sundayis Comic-day.
Mondayis Moan-day.
. . .
Anything else is Party-day!
key {} keys must be hashable (immutable) objects.
Example: Suppose you want to swith on a function and its argument.
Test code (below) returns calculated values using functions like:
def %s(self, *args):\\n\\timport math\\n\\ttmp = (args[0] /
math.pi)\\n\\treturn tmp\\n
or, for humans:
def %s(self, *args):
import math
tmp = (args[0] / math.pi)
return tmp
that produce:
In toplevel: circ.switch(dC,10), d = 3.18309886184
In toplevel: circ.switch(Cd, 3.18), C = 9.99026463842
In toplevel: circ.switch(rC, 5), r = 0.795774715459
In toplevel: circ.switch(Cr, 0.796), C = 5.00141550451
In toplevel: circ.switch(A , 5), A = 78.5398163397
Thanks to Jean-Paul Calderone for his post at
http://mail.python.org/pipermail/python-list/2007-June/446648.html
in response to a question by vasudevrama t
http://mail.python.org/pipermail/python-list/2007-June/446618.html
'''
#import math
class Switch(object):
def __init__(self, keys, vals, base):
self.dictionary = {}
tmpd = {}
for i in range(len(vals)):
func = ''.join([base[0] % keys[i], vals[i], base[1]])
compile(func, '', 'exec')
exec(func, tmpd)
for k, v in tmpd.items():
if k in keys:
self.dictionary[k] = v
def switch(self, key, *args, **kwargs):
try:
result = self.dictionary[key](self, *args, **kwargs)
except KeyError:
result = self.dictionary['de'](self, *args, **kwargs)
return result
if '__main__' == __name__:
'''Case 1: execute a statement.
'''
keys = ('su', 'mo', 'tu', 'we', 'th', 'fr', 'sa', 'de')
vals = ('Sundayis Comic-day.', 'Mondayis Moan-day.',
'Tuesday is Twos-day.', 'Wednesday is Hump-day.',
'Thursday is Shop-day.', 'Fridayis TGIF-day.',
'Saturday is Food-day.', 'Anything else is Party-day!')
fore = "def %s(self, *args):\n\tprint '"
aft = "'\n"
base = (fore, aft)
day = Switch(keys, vals, base)
for k in keys:
try:
day.switch(k)
except TypeError:
print 'key %s %s keys must be hashable (immutable) objects.' % (k,
type(k))
for k in ('xx', 1234, 12.3, {}):
try:
day.switch(k)
except TypeError:
print 'key %s %s keys must be hashable (immutable) objects.' % (k,
type(k))
'''Case 2: execute an expression.
'''
keys = ('dC', 'Cd', 'rC', 'Cr', 'A', 'de')
vals = ("(args[0] / math.pi)", # diameter given Circumference
"(math.pi * args[0])", # Circumferene given diameter
"(args[0] / (2 *
