"Nope... no such luck, there's NO WAY to find out if a variable is defined in python."

It's actually easy in Python:

>>> self = {'foo': 'bar', 'baz': 'bap'}
>>> self.has_key('foo')
True
>>> self.has_key('blarg')
False

For individual variables, rather than dictionary keys, you can test using the same has_key() method, but for the locals() dictionary:

>>> foo = 'bar'
>>> locals().has_key('foo')
True
>>> locals().has_key('baz')
False

Easy, peasy.

It's actually just:

params = {
'foo': 1,
}

if 'foo' in params:
print 'self.params['foo'] ok'

if 'bar' in params:
print 'self.params['bar'] ok'

Related:

http://coffeeghost.net/2008/03/19/your-ignorance-does-not-make-a-programming-language-suck/

@Ryan: that method is not only ugly, but also unreliable: see http://mail.python.org/pipermail/python-list/2002-July/156865.html (since the variable is from an outside scope, it's neither local or global......)

@Alastair: that is certainly better, but not as good as simply testing it directly as in ruby. It should Just Work.

@ Testarossa: Fortunately, python doesn't actually SUCK. Then I wouldn't use it (Perl does suck, so I don't use it). But ruby is better.

Simon,

The problem you cited in your post was testing whether a dictionary contains a key. I mentioned one way, and Alastair mentioned a better one:

>>> params = {'foo': 1}
>>> key1 = 'foo'
>>> key2 = 'bar'
>>> key1 in params
True
>>> key2 in params
False

As far as I can see, this syntax *is* just testing it directly. :)

As for the more general problem of discovering whether a variable has been initialized, thanks for pointing out the problem with the method I gave. It does appear that a try / except block is the only way to be certain.

However, as one Pythonista put it:

"Personally, I find it quite nice
that an ugly architecture (such as one based on whether a variable
is already defined/initialized rather than initializing it at the
start with a unique value and testing for that!) requires an ugly
way of expressing it."

http://www.thescripts.com/forum/thread20887.html

Cheers!

I made a text selection in my web browser, right clicked and Googled for "python dictionary, is a certain key set". Even the number one ("I'm feeling lucky!") link that comes up answers your question.

Tired: Ignorance. Wired: Learning.

>>> params = {'foo': 1}
>>> key1 = 'foo'
>>> key2 = 'bar'
>>> try: print 'key1:', params[key1]
... except KeyError: print 'not set'
...
key1: 1
>>> try: print 'key2:', params[key2]
... except KeyError: print 'not set'
...
key2: not set


Yet another way to do it that is usually my favorite since I believe it is actually faster.

-Sim

Checking for exceptions is a bad way to do it. Python is such a nice language, why doesn't it have support for something like this?

Haha. I was going to post here and say "I feel your pain, Simon" as I regularly bash my head against the wall at the stupidity of PHP when compared to Perl....

... and then I read that you are NOT among the initiated! :) What could you possibly have against Perl???!!!

If you don't like Python's default dict, roll your own...

class MyDict(dict):
def __getitem__(self, key):
if key in self:
return dict.__getitem__(self, key)
return None

d = MyDict()
d[42] = 1729
s = d[42] or "not found..."; print s
s = d[666] or "not found..."; print s

Of course, you run into problems if None is a valid value in your dict - but the real point is that it's dead easy to _make_ Python do what you want!

Maybe Ruby is easier to use here but the numbers still say that Python is more popular :-)

self.params.get('foo')

I've always felt that the ability to do these two is a sign of a fundamentally-flawed language.

if @params['foo']
if defined? @params['foo']

I'd only use Ruby if you paid me (and paid me well at that) because of Perl-inspired features like implicit returns and, if I'm reading those two lines right, implicit variable definition.

My advice is to open up Python's interactive interpreter and type "import this" to pull up the easter egg copy of "The Zen of Python".

The behaviour you applaud and I condemn runs up against the following excerpted lines:

Explicit is better than implicit.
Special cases aren't special enough to break the rules.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one— and preferably only one —obvious way to do it.

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