Dictionaries and dot notation

[Martin Drautzburg]

This may be pretty obvious for most of you:

When I have an object (an instance of a class "Foo") I can access
attributes via dot notation:

aFoo.bar

however when I have a dictionary

aDict = {"bar":"something"}

I have to write

aDict["bar"]

What if I want to create a datastructure that can be used in dot
notation without having to create a class, i.e. because those objects
have no behavior at all?

I know that accessing an instance variable via bracket notation would
really have to be written as:

aFoo.__dict__['bar']

but this does not bring me any further, because I would still have to
plug in that __dict__ thing into my datastructure, which leads us to
the same question as above.

Can anyone tell me what I am missing here?

 

[Stefan Behnel]

This may be pretty obvious for most of you:

When I have an object (an instance of a class "Foo") I can access
attributes via dot notation:

aFoo.bar

however when I have a dictionary

aDict = {"bar":"something"}

I have to write

aDict["bar"]

What if I want to create a datastructure that can be used in dot
notation without having to create a class, i.e. because those objects
have no behavior at all?

I know that accessing an instance variable via bracket notation would
really have to be written as:

aFoo.__dict__['bar']

but this does not bring me any further, because I would still have to
plug in that __dict__ thing into my datastructure, which leads us to
the same question as above.

Can anyone tell me what I am missing here?

This?

http://docs.python.org/ref/attribute-access.html

Stefan

 

[Daniel Nogradi]

This may be pretty obvious for most of you:

When I have an object (an instance of a class "Foo") I can access
attributes via dot notation:

aFoo.bar

however when I have a dictionary

aDict = {"bar":"something"}

I have to write

aDict["bar"]

What if I want to create a datastructure that can be used in dot
notation without having to create a class, i.e. because those objects
have no behavior at all?

I know that accessing an instance variable via bracket notation would
really have to be written as:

aFoo.__dict__['bar']

but this does not bring me any further, because I would still have to
plug in that __dict__ thing into my datastructure, which leads us to
the same question as above.

Can anyone tell me what I am missing here?

What's wrong with creating a dummy class?

class data:
pass

mydata = data( )
mydata.foo = 'foo'
mydata.bar = 'bar'

print mydata.foo
print mydata.bar



Daniel

 

[Daniel Nogradi]

This may be pretty obvious for most of you:

When I have an object (an instance of a class "Foo") I can access
attributes via dot notation:

aFoo.bar

however when I have a dictionary

aDict = {"bar":"something"}

I have to write

aDict["bar"]

What if I want to create a datastructure that can be used in dot
notation without having to create a class, i.e. because those objects
have no behavior at all?

A class inheriting from dict and implementing __getattr__ and
__setattr__ should do the trick...

It can do the trick but one has to be careful with attributes that are
used by dict such as update, keys, pop, etc. Actually it's noted in a
comment at http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/361668
why the whole idea (attribute access of dictionaries) is a bad idea
and I tend to agree.

Daniel

 

[Bruno Desthuilliers]

Martin Drautzburg a écrit :

This may be pretty obvious for most of you:

When I have an object (an instance of a class "Foo") I can access
attributes via dot notation:

aFoo.bar

however when I have a dictionary

aDict = {"bar":"something"}

I have to write

aDict["bar"]

What if I want to create a datastructure that can be used in dot
notation without having to create a class, i.e. because those objects
have no behavior at all?

A class inheriting from dict and implementing __getattr__ and
__setattr__ should do the trick...

 

[Bruno Desthuilliers]

Bruno Desthuilliers a écrit :

Martin Drautzburg a écrit :

This may be pretty obvious for most of you:

When I have an object (an instance of a class "Foo") I can access
attributes via dot notation:

aFoo.bar

however when I have a dictionary
aDict = {"bar":"something"}

I have to write

aDict["bar"]

What if I want to create a datastructure that can be used in dot
notation without having to create a class, i.e. because those objects
have no behavior at all?

A class inheriting from dict and implementing __getattr__ and
__setattr__ should do the trick...

Oh, yes, if you don't care about dict-like behaviour, you can also just:

class Data(object):
def __init__(self, **kw):
self.__dict__.update(kw)

 

[Martin Drautzburg]

mydata = data( )
mydata.foo = 'foo'
mydata.bar = 'bar'

print mydata.foo
print mydata.bar

I am aware of all this.
Okay let me rephrase my question: is there a way of using dot notation
without having to create a class?

 

[Martin Drautzburg]

Daniel Nogradi wrote:

It can do the trick but one has to be careful with attributes that are
used by dict such as update, keys, pop, etc. Actually it's noted in a
comment at
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/361668 why the
whole idea (attribute access of dictionaries) is a bad idea and I tend
to agree.

Oh thank you. So all I have to do is have my object's class implement
__setattr__ and __getattr__, or derive it from a class that does so?
And I could save my "attributes" anywhere within my instance variables.
So I could even add a dictionary whose name does not conflict with what
python uses and whose key/value pairs hold the attributes I want to
access with dot notation and delegate all the python attributes to
their native positions? Oh I see, thats tricky. I still need to be
aware of the builtin stuff one way or the other.

Interesting.

 

[Ben Finney]

Okay let me rephrase my question: is there a way of using dot
notation without having to create a class?

Dot notation, e.g. 'foo.bar', is parsed by the interpreter as "access
the attribute named 'bar' of the object 'foo'". Objects have
attributes either by virtue of the class having them, or the object
getting them assigned after creation.

Can you describe what you would change in the above, or can you
re-word your request based on these facts?

 

[Alex Martelli]

I am aware of all this.
Okay let me rephrase my question: is there a way of using dot notation
without having to create a class?

Sure, all you need to create is an *INSTANCE* of a suitable type or
class. For example:

A module may be appropriate, since it's little more than a "wrapper
around a dict to access items by dot notation".


Alex

 

[Martin Drautzburg]

Sure, all you need to create is an *INSTANCE* of a suitable type or
class. For example:


A module may be appropriate, since it's little more than a "wrapper
around a dict to access items by dot notation".

Thanks, I finally got it. Even your previous example actually does the
trick. I did not notice that I can use a single class (or a module) for
all my datastructures, because I can "plug in" new attributes into the
instance without the class knowing about them.

I was mistaken to believe that I had to know about attributes at the
time of class creation. But your expample does not require that. Should
have read this more carefully.

 

[Gabriel Genellina]

En Mon, 23 Apr 2007 03:14:32 -0300, Martin Drautzburg

I did not notice that I can use a single class (or a module) for
all my datastructures, because I can "plug in" new attributes into the
instance without the class knowing about them.

I was mistaken to believe that I had to know about attributes at the
time of class creation. But your expample does not require that. Should
have read this more carefully.

Welcome to Python and its dynamic nature!

 

[Antoon Pardon]

Daniel Nogradi wrote:



Oh thank you. So all I have to do is have my object's class implement
__setattr__ and __getattr__, or derive it from a class that does so?
And I could save my "attributes" anywhere within my instance variables.
So I could even add a dictionary whose name does not conflict with what
python uses and whose key/value pairs hold the attributes I want to
access with dot notation and delegate all the python attributes to
their native positions? Oh I see, thats tricky. I still need to be
aware of the builtin stuff one way or the other.

Maybe you can do the opposite and create a class that implements
__getitem__ and __setitem__ in function of attribute access.

The following is an example:

class Rec(object):
def __init__(__, **kwargs):
for key,value in kwargs.items():
setattr(__, key, value)

def __getitem__(self, key):
return getattr(self, key)

def __setitem__ (self, key, val):
setattr(self, key, val)


rec = Rec(a=1)

print rec.a
print rec["a"]

rec.b = 2
print rec["b"]

rec["c"] = 3
print rec.c

 

[Bruno Desthuilliers]

Martin Drautzburg a écrit :

Daniel Nogradi wrote:





Oh thank you. So all I have to do is have my object's class implement
__setattr__ and __getattr__, or derive it from a class that does so?
And I could save my "attributes" anywhere within my instance variables.
So I could even add a dictionary whose name does not conflict with what
python uses and whose key/value pairs hold the attributes I want to
access with dot notation and delegate all the python attributes to
their native positions? Oh I see, thats tricky. I still need to be
aware of the builtin stuff one way or the other.

Yeps. FWIW, Daniel is right to raise a warning here, and I should have
think one more minute before posting - since in fact in this case you
don't care about dict-like behaviour.