Robin Becker wrote:
Thierry S. wrote:
Hello.
I would to test the existence of a variable before to use it (like
isset($myVar) in PHP).
I try using "if myVar: ", but there is the error meesage (naturally):
"NameError: name 'myVar' is not defined"
Please, could you tell me what for function exist to test the variable
with
Python?
Regards,
if globals().has_key('myVar'):
....
def f(x):
.... if globals().has_key('x'):
.... print 'x is', x
.... else:
.... print 'x is not defined'
.... f(10)
x is not defined
Before anyone points out locals(), I know all about it.
Testing for the existence of a variable is something to avoid doing.
Variables should be bound unconditionally. There is always a sentinel
value that can be used as a place-holder.
Instead of:
# ...
if not isset('x'):
# initialize x
# use x
use:
x = None
# ...
if x is None:
# initialize x
# use x
If None is a valid value for x to take on, pick another sentinel.
Even better is to pick a legitimate and useful value and just write the
whole thing as:
x = default
# use x
Often this is not possible... but often it is.
If it is really necessary to check for the existence of a variable by
name, for example if the variable name taken from user input, then you
may actually wish to use a dict, instead of variables in your local scope:
d = {}
# Get user input, store it in x
if x not in d:
d[x] = initialize()
# use d[x]
Jp
Jp
