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How to tell the difference between string and list


I need to recognize 'var' and ['var'], usually I would use:

if a.__class__() == '':
elif a.__class__() == []:

But unfortunately in Zope PythonScripts, where I need to use this, one
is not supposed to use underscore-prefixed methods as I just found out.

I figure this is a question general enough to post here (and not on the
Zope list), any clues?


Jan Kokoska

Jul 18 '05 #1
4 1576

Jan Kokoska wrote:

But unfortunately in Zope PythonScripts, where I need to use this, one
is not supposed to use underscore-prefixed methods as I just found out. In Zope you can use same_type():
same_type(a, '')
same_type(a, [])
(See Zope Book -> Advanced Zope Scripting -> Built-in Functions)
I figure this is a question general enough to post here (and not on the
Zope list), any clues?


Jan Kokoska


Jul 18 '05 #2
Jan Kokoska <ko*********@globe.cz> writes:

I need to recognize 'var' and ['var'], usually I would use:

if a.__class__() == '':
elif a.__class__() == []:

That's a very strange way of doing it in any situation!


This is an off-the-top-of-the-head-and-not-quite-sober suggestion,
so is probably technically laughable. I'll see how embarassed I
feel tomorrow morning. -- Patrick Gosling, ucam.comp.misc
Jul 18 '05 #3
Jan Kokoska <ko*********@globe.cz> writes:
I need to recognize 'var' and ['var'], usually I would use:


All the other solutions posted are bad because they fail for
user-defined string-like classes (unless those classes use the new 2.2
features by deriving from str or unicode). As long as your strings
aren't huge:

def isstringlike(x):
try: x+""
except TypeError: return False
else: return True
I think I stole this off Alex Martelli.
Jul 18 '05 #4
bo**@oz.net (Bengt Richter) writes:
On 05 Dec 2003 20:24:02 +0000, jj*@pobox.com (John J. Lee) wrote:
bo**@oz.net (Bengt Richter) writes:
>>> s2 = ['abc','def'][...]>
>>> s2 += 'ghi'
>>> s2
['abc', 'def', 'g', 'h', 'i']

That's not what the code I posted tests for.

You mean __iadd__? I know, I was just showing ordinary list
behavior, and showed that as well to imply that that's also
something you might want to override in a consistent way if you were
overriding __add__ as in my example.

What? list.__add__(self, "") raises TypeError, which means lists do
work OK with isstringlike().

Don't forget, an object's __add__ could be defined to accept
_anything_ for addition, and a string would just be a generic thing,
whose stringness was really irrelevant to the adding object. Are you
making other assumptions based on isstringlike?
That's true: especially given the sequence-ness of strings, which was
the whole motivation for this function in the first place! In fact,
isstringlike(UserList()) is true. Bah.

If so, I think those assumptions should be an explicit part of the
test, or you are setting yourself up for using the test and assuming
the wrong things about some object. You can say you'll use
exceptions will sort those things out,


Well, sometimes that's just the problem: exceptions *won't* get
raised, because strings are sequences.

I guess it's all depends on the particular case. :-(
Jul 18 '05 #5

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