Re: Enchancement suggestion for argparse: intuit type from default
On 15Mar2012 12:22, Ben Finney ben+pyt...@benfinney.id.au wrote: | Roy Smith r...@panix.com writes: | I'll admit I hadn't considered that, but I don't see it as a major | problem. The type intuition could be designed to only work for types | other than NoneType. | | −1, then. It's growing too many special cases, and is no longer simple | to describe, so that indicates it's probably a bad idea. If `type` is not supplied and `default` is present and not None, `type` shall be the type of `default`. That seems straightforward to me. It's a single sentence, easy to read and understand, and potentially saves a lot of code verbiage (gratuitous type= prarameters). I say gratuitous because unless `default` is a sentinel for no option supplied, the `type` should always match type(default). Or am I wrong about that? Cheers, -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ You only live once in life, but if you do it right, once is enough! - Rob Castro r...@columbia.edu -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On 3/15/12 5:59 AM, Cameron Simpson wrote: On 15Mar2012 12:22, Ben Finneyben+pyt...@benfinney.id.au wrote: | Roy Smithr...@panix.com writes: | I'll admit I hadn't considered that, but I don't see it as a major | problem. The type intuition could be designed to only work for types | other than NoneType. | | −1, then. It's growing too many special cases, and is no longer simple | to describe, so that indicates it's probably a bad idea. If `type` is not supplied and `default` is present and not None, `type` shall be the type of `default`. That seems straightforward to me. It's a single sentence, easy to read and understand, and potentially saves a lot of code verbiage (gratuitous type= prarameters). I say gratuitous because unless `default` is a sentinel for no option supplied, the `type` should always match type(default). Or am I wrong about that? Yes. Not all type(default) types can be called with a string to produce a valid value. Note that type= is really a misnomer. argparse doesn't really want a type object there; it wants a converter function that takes a string to an object. -- Robert Kern I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth. -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On 15Mar2012 10:06, Robert Kern robert.k...@gmail.com wrote: | On 3/15/12 5:59 AM, Cameron Simpson wrote: | On 15Mar2012 12:22, Ben Finneyben+pyt...@benfinney.id.au wrote: | | Roy Smithr...@panix.com writes: | | I'll admit I hadn't considered that, but I don't see it as a major | | problem. The type intuition could be designed to only work for types | | other than NoneType. | | | | −1, then. It's growing too many special cases, and is no longer simple | | to describe, so that indicates it's probably a bad idea. | | If `type` is not supplied and `default` is present and not None, `type` | shall be the type of `default`. | | That seems straightforward to me. [... sentinels aside...] the `type` | should always match type(default). Or am I wrong about that? | | Yes. Not all type(default) types can be called with a string to produce a valid | value. Note that type= is really a misnomer. argparse doesn't really want a | type object there; it wants a converter function that takes a string to an object. Aha. Still, you could change the docs to say you only need type= if type(default) _isn't_ useful as the string-value converter. -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ As your attorney, it is my duty to inform you that it is not important that you understand what I'm doing or why you're paying me so much money. What's important is that you continue to do so. - Hunter S. Thompson's Samoan Attorney -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
In article mailman.665.1331806024.3037.python-l...@python.org, Robert Kern robert.k...@gmail.com wrote: Yes. Not all type(default) types can be called with a string to produce a valid value. Note that type= is really a misnomer. argparse doesn't really want a type object there; it wants a converter function that takes a string to an object. Orthogonal to my original suggestion, I agree that this is misnamed. I'm +1 on the idea of renaming it to conversion= or something like that (we'd need to keep type= around as a deprecated synonym for backwards compatability). It's really hard to get your head around type=open. -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On 15Mar2012 09:28, Roy Smith r...@panix.com wrote: | In article mailman.665.1331806024.3037.python-l...@python.org, | Robert Kern robert.k...@gmail.com wrote: | Yes. Not all type(default) types can be called with a string to produce a | valid | value. Note that type= is really a misnomer. argparse doesn't really want a | type object there; it wants a converter function that takes a string to an | object. | | Orthogonal to my original suggestion, I agree that this is misnamed. | I'm +1 on the idea of renaming it to conversion= or something like that | (we'd need to keep type= around as a deprecated synonym for backwards | compatability). It's really hard to get your head around type=open. factory? Anyway, far too late to change this now! -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ all coders are created equal; that they are endowed with certain unalienable rights, of these are beer, net connectivity, and the pursuit of bugfixes... - Gregory R Block -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On Mar 14, 2:08 am, r...@panix.com (Roy Smith) wrote:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default)) without my having to write:
parser.add_argument('--foo', type=int, default=100)
Does this seem like a reasonable enhancement to argparse?
Sounds reasonable to me
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On Wed, 14 Mar 2012 08:35:12 +1100, Ben Finney wrote:
r...@panix.com (Roy Smith) writes:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default))
[…]
-0.5.
That feels too magical to me. I don't see a need to special-case that
usage. There's not much burden in being explicit for the argument type.
And yet you are programming in Python instead of Java, Pascal or Ada :)
It's not magic at all, it's science! Or to be precise, it's a very simple
form of type inference, similar to (but much more basic than) that used
by languages such as Go, Haskell, Ocaml, and ML.
http://en.wikipedia.org/wiki/Type_inference
Given the premise that arguments in argparser are typed, if the argument
can take the value 100 (the default), it is logical that it can't be a
string (because 100 is not a string) or a boolean (because 100 is not a
boolean) or a list (because... well, you get the point).
What if you want an argument --foo that will accept arbitrary types? Then
you would need some way to tell argparse not to infer the type from the
default.
Python *names* are not typed, but objects are. Python infers the type of
the object from its syntax. We write:
n = 100
and not:
n = int 100
Assuming that argparse arguments are typed, and that there is a way to
over-rule the type-inference, there is no need to declare types in the
common case. Explicit declarations should be used only for the uncommon
cases where type inference cannot cope.
--
Steven
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
Am 13.03.2012 22:08, schrieb Roy Smith:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default)) without my having to write:
parser.add_argument('--foo', type=int, default=100)
Does this seem like a reasonable enhancement to argparse?
The following would turn into an error:
# in foo.py:
p.add_argument('--offset', 0)
# calling foo.py:
foo.py --offset 1.5
OTOH, this would be obvious even from halfway serious testing, so I'm +1
for making this change. Have you looked at existing use of this and
where it would break anything? When the argument doesn't match the type,
is the error message sufficiently understandable?
Uli
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
Steven D'Aprano steve+comp.lang.pyt...@pearwood.info writes: On Wed, 14 Mar 2012 08:35:12 +1100, Ben Finney wrote: That feels too magical to me. I don't see a need to special-case that usage. There's not much burden in being explicit for the argument type. And yet you are programming in Python instead of Java, Pascal or Ada :) That's a good point :-) It's not magic at all, it's science! Or to be precise, it's a very simple form of type inference Right. I dislike proposals for run-time type inference in Python, since they are too magical. Especially since we're talking about user input (arguments from the command line to the program); that requires more explicit declarations and checking, not less. What if you want an argument --foo that will accept arbitrary types? Then you would need some way to tell argparse not to infer the type from the default. So we would then need to special-case the special-case? Even more reason to dislike this proposal. Explicit declarations should be used only for the uncommon cases where type inference cannot cope. That's our point of disagreement, then: I think explicit declarations should be required regarding user input. -- \ “Ridicule is the only weapon which can be used against | `\ unintelligible propositions.” —Thomas Jefferson, 1816-07-30 | _o__) | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
In article 87399bgw18@benfinney.id.au,
Ben Finney ben+pyt...@benfinney.id.au wrote:
Right. I dislike proposals for run-time type inference in Python, since
they are too magical.
Especially since we're talking about user input (arguments from the
command line to the program); that requires more explicit declarations
and checking, not less.
What if you want an argument --foo that will accept arbitrary types? Then
you would need some way to tell argparse not to infer the type from the
default.
So we would then need to special-case the special-case? Even more reason
to dislike this proposal.
Explicit declarations should be used only for the uncommon cases where
type inference cannot cope.
That's our point of disagreement, then: I think explicit declarations
should be required regarding user input.
I wasn't suggesting that the type be inferred from what the user
entered. I was suggesting it be inferred from what the programmer had
done (i.e. what value they had given the 'default' parameter).
It's already inferred that the type is a string if you don't give it any
value. What possible meaning could:
parser.add_argument('--foo', default=100)
have? If I run the program with:
$ prog
then foo defaults to the integer 100, but if I run it with:
$ prog --foo=100
then I get the string 100? Surely there's not much of a use case for
that.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On 14/03/2012 13:30, Roy Smith wrote:
In article87399bgw18@benfinney.id.au,
Ben Finneyben+pyt...@benfinney.id.au wrote:
Right. I dislike proposals for run-time type inference in Python, since
they are too magical.
Especially since we're talking about user input (arguments from the
command line to the program); that requires more explicit declarations
and checking, not less.
What if you want an argument --foo that will accept arbitrary types? Then
you would need some way to tell argparse not to infer the type from the
default.
So we would then need to special-case the special-case? Even more reason
to dislike this proposal.
Explicit declarations should be used only for the uncommon cases where
type inference cannot cope.
That's our point of disagreement, then: I think explicit declarations
should be required regarding user input.
I wasn't suggesting that the type be inferred from what the user
entered. I was suggesting it be inferred from what the programmer had
done (i.e. what value they had given the 'default' parameter).
In other words, if there's a default but no explicit type, then the
type is the type of the default.
It's already inferred that the type is a string if you don't give it any
value. What possible meaning could:
parser.add_argument('--foo', default=100)
have? If I run the program with:
$ prog
then foo defaults to the integer 100, but if I run it with:
$ prog --foo=100
then I get the string 100? Surely there's not much of a use case for
that.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On Wed, Mar 14, 2012 at 7:30 AM, Roy Smith r...@panix.com wrote:
It's already inferred that the type is a string if you don't give it any
value. What possible meaning could:
parser.add_argument('--foo', default=100)
have? If I run the program with:
$ prog
then foo defaults to the integer 100, but if I run it with:
$ prog --foo=100
then I get the string 100? Surely there's not much of a use case for
that.
What about:
parser.add_argument('--foo', default=None)
Probably it should not infer NoneType as the argument type in this
case. So would it just ignore the default in this case and let the
type remain str?
Also, how would the inference interact with different actions? For example:
parser.add_argument('--foo', action='append', default=['one'])
I'm not exactly sure what a use case for this might be, but anyway,
the type here should clearly be str, not list. And then what about
this variation:
parser.add_argument('--foo', action='append', default=[1])
Should it try to infer that because the list contains an int, the type
should be int? And even if you manage to get the inference working
flawlessly and expectedly for append, what about custom actions?
It seems to me that there are a large number of edge cases here that
will end up hurting predictability for the end user.
Cheers,
Ian
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On 3/13/2012 2:08 PM, Roy Smith wrote:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default)) without my having to write:
parser.add_argument('--foo', type=int, default=100)
Does this seem like a reasonable enhancement to argparse?
default=None
presents some problems.
John Nagle
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
In article 4f612a9d$0$12033$742ec...@news.sonic.net,
John Nagle na...@animats.com wrote:
On 3/13/2012 2:08 PM, Roy Smith wrote:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default)) without my having to write:
parser.add_argument('--foo', type=int, default=100)
Does this seem like a reasonable enhancement to argparse?
default=None
presents some problems.
I'll admit I hadn't considered that, but I don't see it as a major
problem. The type intuition could be designed to only work for types
other than NoneType.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
Roy Smith r...@panix.com writes: I'll admit I hadn't considered that, but I don't see it as a major problem. The type intuition could be designed to only work for types other than NoneType. −1, then. It's growing too many special cases, and is no longer simple to describe, so that indicates it's probably a bad idea. -- \ “[W]e are still the first generation of users, and for all that | `\ we may have invented the net, we still don't really get it.” | _o__) —Douglas Adams | Ben Finney -- http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
On 15/03/2012 00:52, Roy Smith wrote:
In article4f612a9d$0$12033$742ec...@news.sonic.net,
John Naglena...@animats.com wrote:
On 3/13/2012 2:08 PM, Roy Smith wrote:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default)) without my having to write:
parser.add_argument('--foo', type=int, default=100)
Does this seem like a reasonable enhancement to argparse?
default=None
presents some problems.
I'll admit I hadn't considered that, but I don't see it as a major
problem. The type intuition could be designed to only work for types
other than NoneType.
True, you could consider that a special case.
If you really do want NoneType, or if the type doesn't otherwise match
the default (or there's no default), then you can still be explicit.
--
http://mail.python.org/mailman/listinfo/python-list
Enchancement suggestion for argparse: intuit type from default
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default)) without my having to write:
parser.add_argument('--foo', type=int, default=100)
Does this seem like a reasonable enhancement to argparse?
--
http://mail.python.org/mailman/listinfo/python-list
Re: Enchancement suggestion for argparse: intuit type from default
r...@panix.com (Roy Smith) writes:
Using argparse, if I write:
parser.add_argument('--foo', default=100)
it seems like it should be able to intuit that the type of foo should
be int (i.e. type(default))
[…]
-0.5.
That feels too magical to me. I don't see a need to special-case that
usage. There's not much burden in being explicit for the argument type.
--
\ “Value your freedom or you will lose it, teaches history. |
`\ “Don't bother us with politics,” respond those who don't want |
_o__)to learn.” —Richard M. Stallman, 2002 |
Ben Finney
--
http://mail.python.org/mailman/listinfo/python-list
