Creating a variable of a specific type

[Dan Williams]

Hi people

I understand that Python is strongly, but also dynamically typed. However, I
need to create a variable which initially isn't actually set to anything,
but still needs to be of a certain type. I've looked around a bit, but I'm
not entirely sure how to do this.

Specifically, I have a bit of C++ code which says:

struct in_addr ipAddress;
struct sockaddr_in serverAddress;
map<int, socklen_t> clientAddressSi zes;
fd_set primarySocketTa ble;
map<int, pthread_t> socketThreads;

....this is an except of a sample of different types that I am using. From
looking at the documentation for the Python socket module, for instance, I
can see that the function socket.inet_nto a() requires a variable in struct
in_addr form, just like in C++. The difference is that I don't know how to
set up a 'blank' variable, if you see what I mean!

I'm sure it's just a simple thing that has eluded me somehow, but in
addition to answering this question if anyone can speak any wisdom about the
other data types I have listed above, and their equivalents in Python, that
would be helpful.

Cheers

Dan

[Richard Brodie]

Re: Creating a variable of a specific type


"Dan Williams" <dan@ithium.net > wrote in message
news:mailman.12 36.1075978481.1 2720.python-list@python.org ...
[color=blue]
> ...this is an except of a sample of different types that I am using. From
> looking at the documentation for the Python socket module, for instance, I
> can see that the function socket.inet_nto a() requires a variable in struct
> in_addr form, just like in C++.[/color]

It would be more helpful to say that inet_ntoa is a way to deal with a
variable in packed format. Unless you are deserializing from a buffer
(e.g for a binary wire protocol), or embedding 'C' code you probably
don't need to use it anyway.

It might be worth investing a few minutes playing with the example
socket programs from the Python docs, even if you are an experienced
programmer: http://www.python.org/doc/current/li...t-example.html

[Diez B. Roggisch]

Re: Creating a variable of a specific type

> I understand that Python is strongly, but also dynamically typed. However,[color=blue]
> I need to create a variable which initially isn't actually set to
> anything, but still needs to be of a certain type. I've looked around a
> bit, but I'm not entirely sure how to do this.
>
> Specifically, I have a bit of C++ code which says:
>
> struct in_addr ipAddress;
> struct sockaddr_in serverAddress;
> map<int, socklen_t> clientAddressSi zes;
> fd_set primarySocketTa ble;
> map<int, pthread_t> socketThreads;
>
> ...this is an except of a sample of different types that I am using. From
> looking at the documentation for the Python socket module, for instance, I
> can see that the function socket.inet_nto a() requires a variable in struct
> in_addr form, just like in C++. The difference is that I don't know how to
> set up a 'blank' variable, if you see what I mean!
>
> I'm sure it's just a simple thing that has eluded me somehow, but in
> addition to answering this question if anyone can speak any wisdom about
> the other data types I have listed above, and their equivalents in Python,
> that would be helpful.[/color]

While your are right that python is strong and dynamically typed, you have a
(very common) misconception about variables in python. In python, you have
values on the one side, and _names_ that are bound to certain values on the
other side. So

a = 10
a = "20"

is perfectly legal - and there is no way of limiting the types of values
bound to a (which is what you are asking for). Now while this might look
like weak typing, its not, as this example shows:

a = 10
b = "30"
a + b
TypeError: unsupported operand type(s) for +: 'i

That would go with perl or tcl - and even C! In c, the example would more
look like this:

int a = 10;
char *b = "30";
a + b;

This compiles without any complaints....

Now back to your problem: I don't know right from my head what socket
requires as inet-type, but I guess its a tuple of some sort. You don't need
to declare a variable for that - just use it. If you need to check for a
symbol not beeing initialized, simply use None as value, like here:

address = None
if address:
do_something(ad dress)

None is considered false. You might be more explicit, by using

if address == None:

but thats a matter of taste (to me, at least...)


--
Regards,

Diez B. Roggisch

[Aahz]

Re: Creating a variable of a specific type

In article <bvtei9$10enm4$ 1@ID-111250.news.uni-berlin.de>,
Diez B. Roggisch <nospam-deets@web.de> wrote:[color=blue]
>
>Now back to your problem: I don't know right from my head what socket
>requires as inet-type, but I guess its a tuple of some sort. You don't need
>to declare a variable for that - just use it. If you need to check for a
>symbol not beeing initialized, simply use None as value, like here:
>
>address = None
>if address:
> do_something(ad dress)
>
>None is considered false. You might be more explicit, by using
>
>if address == None:
>
>but thats a matter of taste (to me, at least...)[/color]

No, that's not a matter of taste, it's a matter of incorrect coding.
Using ``==`` calls a method on address, which could return true even if
address isn't None. Much better to use ``is``, which is guaranteed to
return true only if address really *is* None.

Note that in the absence of special methods for comparison, all Python
objects are true, so your original formulation is especially appropriate.
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

"The joy of coding Python should be in seeing short, concise, readable
classes that express a lot of action in a small amount of clear code --
not in reams of trivial code that bores the reader to death." --GvR

[Jp Calderone]

Re: Creating a variable of a specific type

On Thu, Feb 05, 2004 at 08:54:29AM -0500, Aahz wrote:[color=blue]
> In article <bvtei9$10enm4$ 1@ID-111250.news.uni-berlin.de>,
> Diez B. Roggisch <nospam-deets@web.de> wrote:[color=green]
> >
> >Now back to your problem: I don't know right from my head what socket
> >requires as inet-type, but I guess its a tuple of some sort. You don't need
> >to declare a variable for that - just use it. If you need to check for a
> >symbol not beeing initialized, simply use None as value, like here:
> >
> >address = None
> >if address:
> > do_something(ad dress)
> >
> >None is considered false. You might be more explicit, by using
> >
> >if address == None:
> >
> >but thats a matter of taste (to me, at least...)[/color]
>
> No, that's not a matter of taste, it's a matter of incorrect coding.
> Using ``==`` calls a method on address, which could return true even if
> address isn't None. Much better to use ``is``, which is guaranteed to
> return true only if address really *is* None.
>
> Note that in the absence of special methods for comparison, all Python
> objects are true, so your original formulation is especially appropriate.[/color]

Just as using "==" calls a method on address, which could return true even
if address isn't None, calling bool() with address may return false, even if
address isn't None! "if address:" may work in some cases, but it will
return incorrect results when address has been initialized to another false
value ([] is especially common, I find), when it is initalized to a class
defining __nonzero__/__len__ in certainly ways, and in some unfortunate
cases it may even raise an exception (eg, cgi.FieldStorag e).

Jp

[Terry Reedy]

Re: Creating a variable of a specific type


"Jp Calderone" <exarkun@intarw eb.us> wrote in message
news:2004020514 3140.GA22941@in tarweb.us...[color=blue]
> On Thu, Feb 05, 2004 at 08:54:29AM -0500, Aahz wrote:[color=green]
> > In article <bvtei9$10enm4$ 1@ID-111250.news.uni-berlin.de>,
> > Diez B. Roggisch <nospam-deets@web.de> wrote:[color=darkred]
> > >
> > >Now back to your problem: I don't know right from my head what socket
> > >requires as inet-type, but I guess its a tuple of some sort. You don't[/color][/color][/color]
need[color=blue][color=green][color=darkred]
> > >to declare a variable for that - just use it. If you need to check for[/color][/color][/color]
a[color=blue][color=green][color=darkred]
> > >symbol not beeing initialized, simply use None as value, like here:
> > >
> > >address = None
> > >if address:
> > > do_something(ad dress)
> > >
> > >None is considered false. You might be more explicit, by using
> > >
> > >if address == None:
> > >
> > >but thats a matter of taste (to me, at least...)[/color]
> >
> > No, that's not a matter of taste, it's a matter of incorrect coding.
> > Using ``==`` calls a method on address, which could return true even if
> > address isn't None. Much better to use ``is``, which is guaranteed to
> > return true only if address really *is* None.
> >
> > Note that in the absence of special methods for comparison, all Python
> > objects are true, so your original formulation is especially[/color][/color]
appropriate.[color=blue]
>
> Just as using "==" calls a method on address, which could return true[/color]
even[color=blue]
> if address isn't None, calling bool() with address may return false, even[/color]
if[color=blue]
> address isn't None! "if address:" may work in some cases, but it will
> return incorrect results when address has been initialized to another[/color]
false[color=blue]
> value ([] is especially common, I find), when it is initalized to a class
> defining __nonzero__/__len__ in certainly ways, and in some unfortunate
> cases it may even raise an exception (eg, cgi.FieldStorag e).[/color]

Identity to None should be tested for with the 'is' operator (if address is
None:...), which quickly tests for identity of the two objects. Equality
of value and boolean value are both slower and a bit slippery (type
specific).

Terry J. Reedy