Checking if a variable is a dictionary

[Guillermo]

Hello,

This is my first post here. I'm getting my feet wet with Python and I
need to know how can I check whether a variable is of type dictionary.

Something like this:

if isdict(a) then print "a is a dictionary"

Regards,

Guillermo

[Sam]

Hello

if type(a) is dict:
print "a is a dictionnary!"

++

Sam

[Gabriel Genellina]

En Thu, 06 Mar 2008 10:10:47 -0200, Guillermo
<gu**************@googlemail.comescribi�:

This is my first post here. I'm getting my feet wet with Python and I
need to know how can I check whether a variable is of type dictionary.

Something like this:

if isdict(a) then print "a is a dictionary"

if isinstance(a, dict): ...

But note that it's more common in Python to try to use it in the intended
way, and catch the possible errors.
Look for "duck typing" in this group, and the difference between "easier
to ask forgiveness
than permission" and "look before you leap".

--
Gabriel Genellina

[Jeffrey Seifried]

On Mar 6, 7:10 am, Guillermo <[e-mail]:

Hello,

This is my first post here. I'm getting my feet wet with Python and I
need to know how can I check whether a variable is of type dictionary.

Something like this:

if isdict(a) then print "a is a dictionary"

Regards,

Guillermo

Or

if type(a)==type({}):
print 'a is a dictionary'

[Guillermo]

You can also get the dynamic polymorphism without invoking inheritance

by specifying a protocol that the values in your dict must implement,
instead. Protocols are plentiful in Python, perhaps more popular than
type hierarchies.

I'm used to languages with stricter rules than Python. I've read a bit
about Python protocols, but where could I get some more info on
implementation details? Sounds very interesting.

Regards,

Guillermo

[Neil Cerutti]

On Thu, Mar 6, 2008 at 12:17 PM, Guillermo
<gu**************@googlemail.comwrote:

You can also get the dynamic polymorphism without invoking inheritance
by specifying a protocol that the values in your dict must implement,
instead. Protocols are plentiful in Python, perhaps more popular than
type hierarchies.

I'm used to languages with stricter rules than Python. I've read a bit
about Python protocols, but where could I get some more info on
implementation details? Sounds very interesting.

A protocol is just an interface that an object agrees to implement. In
your case, you would state that every object stored in your special
dict must implement the to_tagged_value method with certain agreeable
semantics.

Python implements sequence types, mapping types, iterators, file
streams, and other things with protocols.

For example, iterators must support the "next" and and "__iter__" methods.

Using a protocol instead instead of a type hierarchy requires less
boring boilerplate, and I suppose in Python will usuallly be
preferable except when you want to inherit implementation as well as
interface. In that case, inheritance often saves boilerplate cide
rather than increases it.

It's also misnomered as duck-typing (clearly it should be nomed quack-typing).

--
Neil Cerutti <mr***************@gmail.com>

[Guillermo]

Mamma mia! My head just exploded. I've seen the light.

So you only need to ·want· to have a protocol? That's amazing... Far
beyond the claim that Python is easy. You define protocols in writing
basically! Even my grandma could have her own Python protocol.

Okay, so I think I know where's the catch now -- you must rely on the
fact that the protocol is implemented, there's no way to enforce it if
you're expecting a parrot-like object. You'd try to call the speak()
method and deal with the error if there's no such method?

Thanks a lot!

Guillermo

[Steven D'Aprano]

On Mon, 10 Mar 2008 14:29:48 +0000, Andrew Koenig wrote:

So the question you need to answer is whether you want to determine
whether an object is exactly of type dict, or whether it you are willing
to accept types derived from dict also.

Or other mappings that don't inherit from dict but behave just like
dicts, such as UserDict.
--
Steven