Test for structure

[alex]

Hi there,

how can I check if a variable is a structure (i.e. a list)? For my
special problem the variable is either a character string OR a list of
character strings line ['word1', 'word2',...]

So how can I test if a variable 'a' is either a single character string
or a list? I tried:

if a is list:
but that does not work. I also looked in the tutorial and used google
to find an answer, but I did not.

Has anyone an idea about that?

Alex

[Diez B. Roggisch]

import types

v = []
if type(v) is types.ListType:
pass
--
Regards,

Diez B. Roggisch

[Simon Brunning]

On Wed, 16 Feb 2005 07:11:08 -0800 (PST), alex
<al************ *@mpi-hd.mpg.de> wrote:

how can I check if a variable is a structure (i.e. a list)? For my
special problem the variable is either a character string OR a list of
character strings line ['word1', 'word2',...]

So how can I test if a variable 'a' is either a single character string
or a list? I tried:

if a is list:

but that does not work. I also looked in the tutorial and used google
to find an answer, but I did not.

Has anyone an idea about that?

<http://www.brunningonl ine.net/simon/blog/archives/001349.html>

--
Cheers,
Simon B,
si***@brunningo nline.net,
http://www.brunningonline.net/simon/blog/

[Chris Cioffi]

Perhaps you're looking for the type() built in function and the types modules?

type('aaa') <type 'str'> type([]) <type 'list'> import types
if type([]) is types.ListType:

.... print 'is a list'
....
is a list

Chris

On Wed, 16 Feb 2005 07:10:56 -0800 (PST), alex
<al************ *@mpi-hd.mpg.de> wrote: Hi there,

how can I check if a variable is a structure (i.e. a list)? For my
special problem the variable is either a character string OR a list of
character strings line ['word1', 'word2',...]

So how can I test if a variable 'a' is either a single character string
or a list? I tried:

if a is list:

but that does not work. I also looked in the tutorial and used google
to find an answer, but I did not.

Has anyone an idea about that?

Alex

--
http://mail.python.org/mailman/listinfo/python-list

--
"It is our responsibilitie s, not ourselves, that we should take
seriously." -- Peter Ustinov

[Steven Bethard]

alex wrote:

So how can I test if a variable 'a' is either a single character string
or a list?

py> def test(x):
.... return (isinstance(x, list) or
.... isinstance(x, basestring) and len(x) == 1)
....
py> test('a')
True
py> test('ab')
False
py> test([])
True
py> test(['a', 'b'])
True

But definitely read Simon Brunning's post - you probably don't actually
want to do this test. Why do you think you want to test this? What's
your use case?

STeVe

[Michael Hartl]

I use a function isListLike in cases such as this one:

# def isListLike(L):
# """Return True if L is list-like, False otherwise."""
# try:
# L + []
# return True
# except:
# return False

Then you can use a standard if-else construct:

# if isListLike(myva r):
# <do something>
# else:
# <do something else>

Michael

--
Michael D. Hartl, Ph.D.
Chief Technology Officer
http://quarksports.com/

[Steven Bethard]

Michael Hartl wrote:

I use a function isListLike in cases such as this one:

# def isListLike(L):
# """Return True if L is list-like, False otherwise."""
# try:
# L + []
# return True
# except:
# return False

Then you can use a standard if-else construct:

# if isListLike(myva r):
# <do something>
# else:
# <do something else>

What kind of situations do you use this for? I almost never have to do
this kind of typechecking. If it's supposed to be a list, I just use it
as a list...

STeVe

[Ben Finney]

alex wrote On 17/02/05 02:08:

how can I check if a variable is a structure (i.e. a list)? For my
special problem the variable is either a character string OR a list
of character strings line ['word1', 'word2',...]

You're trying to apply the LBYL principle. My bet is that your "special
problem" can be solved by the EAFP principle. (These terms are
explained in the Glossary of the tutorial,
<http://www.python.org/doc/current/tut/node18.html>.)

If you test for a specific set of types, your code will not work with
types that you have not considered yet behave like lists. As you
discover more object types that you want the code to work with, it will
sprout more cruft for checking those types.

If you want to use some list behaviour of the object, don't check first.
Use it, and catch the TypeError exception in the event that it's not.
This way, *any* object that implements the functionality you need will
work, regardless of its type.

[Terry Hancock]

On Wednesday 16 February 2005 09:08 am, alex wrote:

how can I check if a variable is a structure (i.e. a list)? For my
special problem the variable is either a character string OR a list of
character strings line ['word1', 'word2',...]

So how can I test if a variable 'a' is either a single character string
or a list?

The literally correct but actually wrong answer is:

if type(a) == type([]):
print "'a' is a duck"

But you probably shouldn't do that. You should probably just test to
see if the object is iterable --- does it have an __iter__ method?

Which might look like this:

if hasattr(a, '__iter__'):
print "'a' quacks like a duck"

That way your function will also work if a happens to be a tuple,
a dictionary, or a user-defined class instance which is happens to
be iterable.

Being "iterable" means that code like:

for i in a:
print "i=%s is an element of a" % repr(i)

works. Which is probably why you wanted to know, right?

Cheers,
Terry

--
--
Terry Hancock ( hancock at anansispacework s.com )
Anansi Spaceworks http://www.anansispaceworks.com

[Martin Miller]

I don't believe you can use the test for a __iter__ attribute in this
case, for the following reason:

c1 = 'abc'
c2 = ['de', 'fgh', 'ijkl']
hasattr(c1, '__iter__') False hasattr(c2, '__iter__') True for i in c1: print "i=%s is an element of c1" % repr(i)

....
i='a' is an element of c1
i='b' is an element of c1
i='c' is an element of c1

In other words, even though the c1 single string variable does not have
an __iter__ attribute, it can still be used in a for loop. I think the
right answer would depend on what exactly the OP intends to do with the
argument when it is a list (or is list-like in some way) -- i.e. he
didn't say specifically that he wanted use it in a for loop.

-Martin
=============== =====
Terry Hancock wrote:

On Wednesday 16 February 2005 09:08 am, alex wrote:

how can I check if a variable is a structure (i.e. a list)? For my
special problem the variable is either a character string OR a list

of character strings line ['word1', 'word2',...]

So how can I test if a variable 'a' is either a single character string or a list?

The literally correct but actually wrong answer is:

if type(a) == type([]):
print "'a' is a duck"

But you probably shouldn't do that. You should probably just test to
see if the object is iterable --- does it have an __iter__ method?

Which might look like this:

if hasattr(a, '__iter__'):
print "'a' quacks like a duck"

That way your function will also work if a happens to be a tuple,
a dictionary, or a user-defined class instance which is happens to
be iterable.

Being "iterable" means that code like:

for i in a:
print "i=%s is an element of a" % repr(i)

works. Which is probably why you wanted to know, right?

Cheers,
Terry

--
--
Terry Hancock ( hancock at anansispacework s.com )
Anansi Spaceworks http://www.anansispaceworks.com