Re: Built-in functions and keyword arguments
On Tue, 30 Oct 2007 07:28:07 +0200, Hendrik van Rooyen wrote: yes - the point I am trying to make is that the intention of the OP was to use an assignment as an argument, and you can't do that, as the interpreter thinks its a keyword. Hence the gotcha. Then you must have misunderstand his intention. He wanted to give the optional third argument of `getattr()` as keyword argument. Ciao, Marc 'BlackJack' Rintsch -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
J. Clifford Dyer [EMAIL PROTECTED] wrote:
How do you interpret:
help(__import__)
Help on built-in function __import__ in module __builtin__:
__import__(...)
__import__(name, globals={}, locals={}, fromlist=[], level=-1) -
module
...
help(int)
Help on class int in module __builtin__:
class int(object)
| int(x[, base]) - integer
...
Can you find any case (other than a single parameter identified as
'object') where you can interpret the help string as telling you the
types of the parameters?
OK, good point. Perhaps it's not so specific as the type, but
certainly the use of name and x in the docstrings listed above only
imply something about the character of the argument, not the name of
the argument itself, which is what I was trying to get at. Help
documentation for keyword arguments usually shows the argument being
used as a keyword, like the example from __import__ above.
Usually, but not in the second example I gave:
int(base=12, x='42')
50
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Re: Built-in functions and keyword arguments
En Tue, 30 Oct 2007 14:43:48 -0300, Duncan Booth [EMAIL PROTECTED] escribió: J. Clifford Dyer [EMAIL PROTECTED] wrote: help(int) Help on class int in module __builtin__: class int(object) | int(x[, base]) - integer ... OK, good point. Perhaps it's not so specific as the type, but certainly the use of name and x in the docstrings listed above only imply something about the character of the argument, not the name of the argument itself, which is what I was trying to get at. Help documentation for keyword arguments usually shows the argument being used as a keyword, like the example from __import__ above. Usually, but not in the second example I gave: int(base=12, x='42') 50 So one can conclude that, unfortunately, there is no way to tell from the docs alone whether a certain function will accept keyword arguments or not. Too bad :( -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
Armando Serrano Lombillo [EMAIL PROTECTED] wrote: Why does Python give an error when I try to do this: len(object=[1,2]) Traceback (most recent call last): File pyshell#40, line 1, in module len(object=[1,2]) TypeError: len() takes no keyword arguments but not when I use a normal function: def my_len(object): return len(object) my_len(object=[1,2]) 2 At the C level there are several options for how you define a function callable from Python. The most general form is METH_VARARGS|METH_KEYWORDS which accepts both a tuple of arguments and a dictionary of keyword arguments. These then have to be parsed to find the actual arguments. Many of the builtin functions use only METH_VARARGS which means they don't support keyword arguments. len is even simpler and uses an option METH_O which means it gets a single object as an argument. This keeps the code very simple and may also make a slight difference to performance. I don't know if the reason that most builtin functions don't accept keywords is just historical (someone would have to go through a lot of code and add keyword argument names) or if there really is any measurable performance difference to not using the METH_KEYWORDS option. Being able to write less C code may be the main factor. -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
Armando Serrano Lombillo a écrit : Why does Python give an error when I try to do this: len(object=[1,2]) Traceback (most recent call last): File pyshell#40, line 1, in module len(object=[1,2]) TypeError: len() takes no keyword arguments but not when I use a normal function: def my_len(object): return len(object) my_len(object=[1,2]) 2 In the second case, the name of the argument *is* 'object'. Which is not the case for the builtin len (which, fwiw, has type 'builtin_function_or_method', not 'function', so inspect.getargspec couldn't tell me more). ot While we're at it, you should avoid using builtin's names for identifiers - here, using 'object' as the arg name shadows the builtin 'object' class). /ot -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
On Mon, 29 Oct 2007 13:52:04 +, Armando Serrano Lombillo wrote: Why does Python give an error when I try to do this: len(object=[1,2]) Traceback (most recent call last): File pyshell#40, line 1, in module len(object=[1,2]) TypeError: len() takes no keyword arguments but not when I use a normal function: def my_len(object): return len(object) my_len(object=[1,2]) 2 Because len() takes no keyword arguments, just like it says, but my_len() is written so it DOES take a keyword argument. When you call a function foo(object=[1,2]) you are instructing Python to bind the value [1,2] to the keyword argument object. But if the function doesn't have a keyword argument named object then it will fail immediately. Since len() doesn't have ANY keyword arguments, naturally it doesn't have one called object. You can see the same effect here: def my_len2(*args): ... return len(arg[0]) ... my_len2(object=[1,2]) Traceback (most recent call last): File stdin, line 1, in module TypeError: my_len2() got an unexpected keyword argument 'object' Apart from the error message being slightly different, many (most? all?) of the built in functions are like my_len2(). You may be thinking that keyword arguments are the equivalent of this: object=[1,2] len(object) That is not the case. They are not at all equivalent. -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
Bruno Desthuilliers [EMAIL PROTECTED] wrote: In the second case, the name of the argument *is* 'object'. Which is not the case for the builtin len (which, fwiw, has type 'builtin_function_or_method', not 'function', so inspect.getargspec couldn't tell me more). ot While we're at it, you should avoid using builtin's names for identifiers - here, using 'object' as the arg name shadows the builtin 'object' class). /ot I think you are being a little bit unfair here: help(len) says: len(...) len(object) - integer Return the number of items of a sequence or mapping. which implies that the argument to len has the name 'object' (although in fact it doesn't have a name). The OP was simply asking about the difference in calling conventions, not proposing to write code using 'object' as the argument name. -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
ot While we're at it, you should avoid using builtin's names for identifiers - here, using 'object' as the arg name shadows the builtin 'object' class). /ot I think you are being a little bit unfair here: help(len) says: len(...) len(object) - integer Return the number of items of a sequence or mapping. which implies that the argument to len has the name 'object' (although in fact it doesn't have a name). And to confound matters even further for the uninitiated, http://docs.python.org/lib/built-in-funcs.html#l2h-45 says that it's len(s) instead (but len(s=[]) doesn't work either) -tkc -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
On Oct 29, 3:10 pm, Duncan Booth [EMAIL PROTECTED] wrote: Armando Serrano Lombillo [EMAIL PROTECTED] wrote: Why does Python give an error when I try to do this: len(object=[1,2]) Traceback (most recent call last): File pyshell#40, line 1, in module len(object=[1,2]) TypeError: len() takes no keyword arguments but not when I use a normal function: def my_len(object): return len(object) my_len(object=[1,2]) 2 At the C level there are several options for how you define a function callable from Python. The most general form is METH_VARARGS|METH_KEYWORDS which accepts both a tuple of arguments and a dictionary of keyword arguments. These then have to be parsed to find the actual arguments. Many of the builtin functions use only METH_VARARGS which means they don't support keyword arguments. len is even simpler and uses an option METH_O which means it gets a single object as an argument. This keeps the code very simple and may also make a slight difference to performance. I don't know if the reason that most builtin functions don't accept keywords is just historical (someone would have to go through a lot of code and add keyword argument names) or if there really is any measurable performance difference to not using the METH_KEYWORDS option. Being able to write less C code may be the main factor. Ok. I was suspecting something like this. Performance issues aside, I think it would be a good idea if built-in functions behaved exactly the same as normal functions. BTW, I came into this problem when trying to use functools.partial: import functools getattrF = functools.partial(getattr, default=False) which I think should have worked if getattr behaved as normal functions do. In the end I did: def getattrF(object, name): return getattr(object, name, False) Any better idea? -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
On Oct 29, 3:20 pm, Bruno Desthuilliers bruno. [EMAIL PROTECTED] wrote: Armando Serrano Lombillo a écrit : Why does Python give an error when I try to do this: len(object=[1,2]) Traceback (most recent call last): File pyshell#40, line 1, in module len(object=[1,2]) TypeError: len() takes no keyword arguments but not when I use a normal function: def my_len(object): return len(object) my_len(object=[1,2]) 2 In the second case, the name of the argument *is* 'object'. Which is not the case for the builtin len (which, fwiw, has type 'builtin_function_or_method', not 'function', so inspect.getargspec couldn't tell me more). so that's the point, built-in functions don't behave as normal functions. ot While we're at it, you should avoid using builtin's names for identifiers - here, using 'object' as the arg name shadows the builtin 'object' class). /ot As Duncan points out, I was using the name object because it's what you get when you type help(len) (or in the calltips, or in). Anyway, I don't think there's any harm in overriding a builtin name in such a small scope (the object=[1,2] won't shadow the built-in object outside of the function). -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
On Mon, 29 Oct 2007 08:34:58 -0700, Armando Serrano Lombillo wrote: On Oct 29, 3:10 pm, Duncan Booth [EMAIL PROTECTED] wrote: I don't know if the reason that most builtin functions don't accept keywords is just historical (someone would have to go through a lot of code and add keyword argument names) or if there really is any measurable performance difference to not using the METH_KEYWORDS option. Being able to write less C code may be the main factor. Ok. I was suspecting something like this. Performance issues aside, I think it would be a good idea if built-in functions behaved exactly the same as normal functions. As Steven D'Aprano showed they behave like normal functions. Even pure Python functions can have arguments without names: def spam(*args): pass Ciao, Marc 'BlackJack' Rintsch -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
Tim Chase [EMAIL PROTECTED] I think you are being a little bit unfair here: help(len) says: len(...) len(object) - integer Return the number of items of a sequence or mapping. which implies that the argument to len has the name 'object' (although in fact it doesn't have a name). And to confound matters even further for the uninitiated, http://docs.python.org/lib/built-in-funcs.html#l2h-45 says that it's len(s) instead (but len(s=[]) doesn't work either) Looks like a gotcha to me - its the difference between a keyword (master = 42) and an assignment (s='I am a string') You just can't do that - how is the parser supposed to know that the second one is an assignment and not a keyword? len([]) should work, though. - Hendrik -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
On Mon, 29 Oct 2007 19:03:34 +0200, Hendrik van Rooyen [EMAIL PROTECTED] wrote: Tim Chase [EMAIL PROTECTED] I think you are being a little bit unfair here: help(len) says: len(...) len(object) - integer Return the number of items of a sequence or mapping. which implies that the argument to len has the name 'object' (although in fact it doesn't have a name). And to confound matters even further for the uninitiated, http://docs.python.org/lib/built-in-funcs.html#l2h-45 says that it's len(s) instead (but len(s=[]) doesn't work either) Looks like a gotcha to me - its the difference between a keyword (master = 42) and an assignment (s='I am a string') But it's not a keyword: len(s=[]) Traceback (most recent call last): File stdin, line 1, in ? TypeError: len() takes no keyword arguments I think that's the issue here. Jean-Paul -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
Armando Serrano Lombillo a écrit : On Oct 29, 3:20 pm, Bruno Desthuilliers bruno. [EMAIL PROTECTED] wrote: Armando Serrano Lombillo a écrit : Why does Python give an error when I try to do this: len(object=[1,2]) Traceback (most recent call last): File pyshell#40, line 1, in module len(object=[1,2]) TypeError: len() takes no keyword arguments but not when I use a normal function: def my_len(object): return len(object) my_len(object=[1,2]) 2 In the second case, the name of the argument *is* 'object'. Which is not the case for the builtin len (which, fwiw, has type 'builtin_function_or_method', not 'function', so inspect.getargspec couldn't tell me more). so that's the point, built-in functions don't behave as normal functions. ot While we're at it, you should avoid using builtin's names for identifiers - here, using 'object' as the arg name shadows the builtin 'object' class). /ot As Duncan points out, I was using the name object because it's what you get when you type help(len) That what I thought. But anyway, you're not the only person reading this newsgroup, and there are a couple gotchas that are worth pointing out for those who don't know yet. (or in the calltips, or in). Anyway, I don't think there's any harm in overriding a builtin name in such a small scope In this specific case, possibly - because there are few chances you need to get at the builtin object type here. Still, it's better to avoid shadowing builtin names IMHO - if only for readability. -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
On Mon, Oct 29, 2007 at 02:27:50PM +, Duncan Booth wrote regarding Re: Built-in functions and keyword arguments: Bruno Desthuilliers [EMAIL PROTECTED] wrote: In the second case, the name of the argument *is* 'object'. Which is not the case for the builtin len (which, fwiw, has type 'builtin_function_or_method', not 'function', so inspect.getargspec couldn't tell me more). ot While we're at it, you should avoid using builtin's names for identifiers - here, using 'object' as the arg name shadows the builtin 'object' class). /ot I think you are being a little bit unfair here: help(len) says: len(...) len(object) - integer Return the number of items of a sequence or mapping. which implies that the argument to len has the name 'object' (although in fact it doesn't have a name). The OP was simply asking about the difference in calling conventions, not proposing to write code using 'object' as the argument name. Hmm To my mind, that just implies that the *type* of the expected input is an object. Just like the - integer tells you that the type of the output is an integer. If the documentation read len(s=object) - integer, then I would expect a keyword argument s typed as an object. Now that might also be misleading, as not all objects have a length (integers, for example raise a TypeError). But the documentation has to say something, which doesn't imply that every argument has to be a keyword. Cheers, Cliff -- http://mail.python.org/mailman/listinfo/python-list
Re: Built-in functions and keyword arguments
J. Clifford Dyer [EMAIL PROTECTED] wrote:
I think you are being a little bit unfair here: help(len) says:
len(...)
len(object) - integer
Return the number of items of a sequence or mapping.
which implies that the argument to len has the name 'object'
(although in fact it doesn't have a name). The OP was simply asking
about the difference in calling conventions, not proposing to write
code using 'object' as the argument name.
Hmm To my mind, that just implies that the *type* of the expected
input is an object. Just like the - integer tells you that the
type of the output is an integer. If the documentation read
len(s=object) - integer, then I would expect a keyword argument s
typed as an object.
How do you interpret:
help(__import__)
Help on built-in function __import__ in module __builtin__:
__import__(...)
__import__(name, globals={}, locals={}, fromlist=[], level=-1) -
module
...
help(int)
Help on class int in module __builtin__:
class int(object)
| int(x[, base]) - integer
...
Can you find any case (other than a single parameter identified as
'object') where you can interpret the help string as telling you the types
of the parameters?
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Re: Built-in functions and keyword arguments
On Mon, Oct 29, 2007 at 06:45:22PM +, Duncan Booth wrote regarding Re:
Built-in functions and keyword arguments:
J. Clifford Dyer [EMAIL PROTECTED] wrote:
I think you are being a little bit unfair here: help(len) says:
len(...)
len(object) - integer
Return the number of items of a sequence or mapping.
which implies that the argument to len has the name 'object'
(although in fact it doesn't have a name). The OP was simply asking
about the difference in calling conventions, not proposing to write
code using 'object' as the argument name.
Hmm To my mind, that just implies that the *type* of the expected
input is an object. Just like the - integer tells you that the
type of the output is an integer. If the documentation read
len(s=object) - integer, then I would expect a keyword argument s
typed as an object.
How do you interpret:
help(__import__)
Help on built-in function __import__ in module __builtin__:
__import__(...)
__import__(name, globals={}, locals={}, fromlist=[], level=-1) -
module
...
help(int)
Help on class int in module __builtin__:
class int(object)
| int(x[, base]) - integer
...
Can you find any case (other than a single parameter identified as
'object') where you can interpret the help string as telling you the types
of the parameters?
OK, good point. Perhaps it's not so specific as the type, but certainly the
use of name and x in the docstrings listed above only imply something about the
character of the argument, not the name of the argument itself, which is what I
was trying to get at. Help documentation for keyword arguments usually shows
the argument being used as a keyword, like the example from __import__ above.
Thanks for the correction.
Cheers,
Cliff
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Re: Built-in functions and keyword arguments
Jean-Paul Calderone [EMAIL PROTECTED] wrote: On Mon, 29 Oct 2007 19:03:34 +0200, Hendrik van Rooyen [EMAIL PROTECTED] wrote: Looks like a gotcha to me - its the difference between a keyword (master = 42) and an assignment (s='I am a string') But it's not a keyword: len(s=[]) Traceback (most recent call last): File stdin, line 1, in ? TypeError: len() takes no keyword arguments I think that's the issue here. yes - the point I am trying to make is that the intention of the OP was to use an assignment as an argument, and you can't do that, as the interpreter thinks its a keyword. Hence the gotcha. - Hendrik -- http://mail.python.org/mailman/listinfo/python-list
