Re: Default parameter for a method
On Apr 16, 4:21 pm, John Nagle <[EMAIL PROTECTED]> wrote:
> In general, default values should be immutable constants only.
This is more restrictive than necessary; it should rather read "In
general, default values should be *treated as* immutable objects
only". It's perfectly fine for a default value to be mutable if the
function doesn't modify it, as in the following example:
def parse(text, stopwords=set(w.strip() for w in
open('stopwords.txt')):
words = [w for w in text.split() if w not in stopwords]
...
Since the set is not modified, there's no harm for being mutable; IOW
it's no different than using a frozenset instead. Similarly for dicts,
lists and other mutable containers, as long as they are treated as
read-only.
George
--
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Re: Default parameter for a method
"John Nagle" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] | [EMAIL PROTECTED] wrote: | > I wanted to know if there's any way to create a method that takes a | > default parameter, and that parameter's default value is the return | > value of another method of the same class. For example: | > | ... | | > | > def meth2(self, arg=meth1()): | |Not good. If the default value of an argument is mutable, there | are wierd effects, because the default value is bound once when the | class is created, then shared between all later uses. This is almost | never what was wanted or intended, and it's a common source of subtle | bugs. | |In general, default values should be immutable constants only. Then one would have to restrict default args to immutable builtins. There is no way to determine (without reading code) whether instances of a user-defined class are mutable or not. tjr -- http://mail.python.org/mailman/listinfo/python-list
Re: Default parameter for a method
[EMAIL PROTECTED] wrote: > I wanted to know if there's any way to create a method that takes a > default parameter, and that parameter's default value is the return > value of another method of the same class. For example: > ... > > def meth2(self, arg=meth1()): Not good. If the default value of an argument is mutable, there are wierd effects, because the default value is bound once when the class is created, then shared between all later uses. This is almost never what was wanted or intended, and it's a common source of subtle bugs. In general, default values should be immutable constants only. There's been talk of fixing this (it's really a design bug in Python), but for now, it's still broken. (I just had horrible thoughts about the implications of binding a closure to a default argument. You don't want to go there.) John Nagle -- http://mail.python.org/mailman/listinfo/python-list
Re: Default parameter for a method
Cliff Wells wrote: > > On Wed, 2008-04-16 at 13:47 -0500, Larry Bates wrote: >> [EMAIL PROTECTED] wrote: >> > I wanted to know if there's any way to create a method that takes a >> > default parameter, and that parameter's default value is the return >> > value of another method of the same class. For example: >> > >> > class A: >> > def __init__(self): >> > self.x = 1 >> > >> > def meth1(self): >> > return self.x >> > >> > def meth2(self, arg=meth1()): >> > # The default `arg' should would take the return value of >> > meth1() >> > print '"arg" is', arg >> > >> > This obviously doesn't work. I know I could do >> > >> > ... >> > def meth2(self, arg=None): >> > if arg is None: >> > arg = self.meth1() >> > >> > but I'm looking for a more straightforward way. >> >> You can write this as: >> >> def meth2(self, arg=None): >> arg = arg or self.meth1() >> >> IMHO - You can't get much more "straightforward" than that. > > What if arg is 0 an empty list or anything else that's "False"? > > def meth2(self, arg=None): > arg = (arg is not None) or self.meth1() > > is what you want. No, it's not: >>> for arg in None, 0, "yadda": ... print "---", arg, "---" ... if arg is None: arg = "call method" ... print "OP:", arg ... print "Larry:", arg or "call method" ... print "Cliff:", (arg is not None) or "call method" ... --- None --- OP: call method Larry: call method Cliff: True --- 0 --- OP: 0 Larry: call method Cliff: True --- yadda --- OP: yadda Larry: yadda Cliff: True Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: Default parameter for a method
On Wed, 2008-04-16 at 13:47 -0500, Larry Bates wrote: > [EMAIL PROTECTED] wrote: > > I wanted to know if there's any way to create a method that takes a > > default parameter, and that parameter's default value is the return > > value of another method of the same class. For example: > > > > class A: > > def __init__(self): > > self.x = 1 > > > > def meth1(self): > > return self.x > > > > def meth2(self, arg=meth1()): > > # The default `arg' should would take the return value of > > meth1() > > print '"arg" is', arg > > > > This obviously doesn't work. I know I could do > > > > ... > > def meth2(self, arg=None): > > if arg is None: > > arg = self.meth1() > > > > but I'm looking for a more straightforward way. > > You can write this as: > > def meth2(self, arg=None): > arg = arg or self.meth1() > > IMHO - You can't get much more "straightforward" than that. What if arg is 0 an empty list or anything else that's "False"? def meth2(self, arg=None): arg = (arg is not None) or self.meth1() is what you want. Regards, Cliff -- http://mail.python.org/mailman/listinfo/python-list
Re: Default parameter for a method
[EMAIL PROTECTED] wrote: > I wanted to know if there's any way to create a method that takes a > default parameter, and that parameter's default value is the return > value of another method of the same class. For example: > > class A: > def __init__(self): > self.x = 1 > > def meth1(self): > return self.x > > def meth2(self, arg=meth1()): > # The default `arg' should would take the return value of > meth1() > print '"arg" is', arg > > This obviously doesn't work. I know I could do > > ... > def meth2(self, arg=None): > if arg is None: > arg = self.meth1() > > but I'm looking for a more straightforward way. You can write this as: def meth2(self, arg=None): arg = arg or self.meth1() IMHO - You can't get much more "straightforward" than that. -Larry -- http://mail.python.org/mailman/listinfo/python-list
Re: Default parameter for a method... again
On Apr 16, 9:26 am, [EMAIL PROTECTED] wrote: > I had posted this before but all the spam whipped it out... > > I wanted to know if there's any way to create a method that takes a > default parameter, and that parameter's default value is the return > value of another method of the same class. For example: > > class A: > def __init__(self): > self.x = 1 > > def meth1(self): > return self.x > > def meth2(self, arg=meth1()): > # The default `arg' should would take thereturn value of > meth1() > print '"arg" is', arg > > This obviously doesn't work. I know I could do > > ... > def meth2(self, arg=None): > if arg is None: > arg = self.meth1() > > but I'm looking for a more straightforward way. That is the straightforward way. It may not seem that way now but all languages have patterns and this is a common one in python. You will see code like this all over Python, even in the standard library. The best thing to do is embrace it. It will not only work, but make your code more readable to others. Matt -- http://mail.python.org/mailman/listinfo/python-list
Default parameter for a method... again
I had posted this before but all the spam whipped it out... I wanted to know if there's any way to create a method that takes a default parameter, and that parameter's default value is the return value of another method of the same class. For example: class A: def __init__(self): self.x = 1 def meth1(self): return self.x def meth2(self, arg=meth1()): # The default `arg' should would take thereturn value of meth1() print '"arg" is', arg This obviously doesn't work. I know I could do ... def meth2(self, arg=None): if arg is None: arg = self.meth1() but I'm looking for a more straightforward way. -- http://mail.python.org/mailman/listinfo/python-list
Default parameter for a method
I wanted to know if there's any way to create a method that takes a default parameter, and that parameter's default value is the return value of another method of the same class. For example: class A: def __init__(self): self.x = 1 def meth1(self): return self.x def meth2(self, arg=meth1()): # The default `arg' should would take the return value of meth1() print '"arg" is', arg This obviously doesn't work. I know I could do ... def meth2(self, arg=None): if arg is None: arg = self.meth1() but I'm looking for a more straightforward way. -- http://mail.python.org/mailman/listinfo/python-list
