What value should be passed to make a function use the default argument value?

[LaundroMat]

Suppose I have this function:

def f(var=1):
return var*2

What value do I have to pass to f() if I want it to evaluate var to 1?
I know that f() will return 2, but what if I absolutely want to pass a
value to f()? "None" doesn't seem to work..

Thanks in advance.

[Fredrik Lundh]

Georg Brandl wrote:

>But that can only work if you are the author of f. Take the
following code:

def myrepeat(obj, times = xxx):
return itertools.repea t(obj, times)

What value do I have to substitue for xxx, so that myrepeat
will have the exact same function as itertools.repea t?

There's no possible value. You'll have to write this like

def myrepeat(obj, times=None):
if times is None:
return itertools.repea t(obj)
else:
return itertools.repea t(obj, times)

or:

def myrepeat(*args) :
return itertools.repea t(*args)

</F>

[Antoon Pardon]

On 2006-10-04, Georg Brandl <g.************ *@gmx.netwrote:

Antoon Pardon wrote:

>On 2006-10-04, Paul Rubin <httpwrote:

>>Antoon Pardon <ap*****@forel. vub.ac.bewrites :
Now in this case you could start by assigning arg the value 1 and
eliminate the if test. However that only works if you know the
default value for the argument. What he seems to be asking for
is if there is an object, (let as call it Default), that would
make code like:

def f(var=1):

Equivallen t to:

def f(var=Default)
if var is Default)
var = 1

Oh, I see. Yes, the OP should just use a distinct default value
instead of 1. I usually do this with

sentinel = object()

def f(var=sentinel) :
if var is sentinel:
# f was called without an arg

But that can only work if you are the author of f. Take the
following code:

def myrepeat(obj, times = xxx):
return itertools.repea t(obj, times)

What value do I have to substitue for xxx, so that myrepeat
will have the exact same function as itertools.repea t?

There's no possible value. You'll have to write this like

Yes, that was the point I wanted to make.

def myrepeat(obj, times=None):
if times is None:
return itertools.repea t(obj)
else:
return itertools.repea t(obj, times)

Many functions implemented in C have this behavior.

Which is a pity and IMO makes the documentation of these
functions a bit problematic. Take the itertool.repeat
documentation:

repeat(object[, times])
Make an iterator that returns object over and over again. Runs
indefinitely unless the times argument is specified. ...

My first impression from this, is that it is possible to call
this as follows:

repeat(None, times = 5)

But that doesn't work either.

For all functions written in Python, you can look up the default
value in the source.

That wont help much if you would like something like the following:

def fun(f):

arg = Default
try:
arg = Try_Processing( )
except Nothing_To_Proc ess:
pass
f(arg)

--
Antoon Pardon

[Antoon Pardon]

On 2006-10-04, Fredrik Lundh <fr*****@python ware.comwrote:

Georg Brandl wrote:

>>But that can only work if you are the author of f. Take the
following code:

def myrepeat(obj, times = xxx):
return itertools.repea t(obj, times)

What value do I have to substitue for xxx, so that myrepeat
will have the exact same function as itertools.repea t?

There's no possible value. You'll have to write this like

def myrepeat(obj, times=None):
if times is None:
return itertools.repea t(obj)
else:
return itertools.repea t(obj, times)

or:

def myrepeat(*args) :
return itertools.repea t(*args)

Yes that works but I have the impression that this solution
becomes complicated very fast once you want to do extra
processing in the function body. Take the following

def myrepeat(obj, times = xxx)"
newobj = Process(obj)
return itertools.repea t(obj, times)

I think it would become something like:

def myrepeat(*args) :
obj = args[0]
tail = args[1:]
newobj = Process(obj)
newargs = (newobj,) + tail
return itertools.repea t(*newargs)

[Paul Rubin]

Antoon Pardon <ap*****@forel. vub.ac.bewrites :

repeat(object[, times])
Make an iterator that returns object over and over again. Runs
indefinitely unless the times argument is specified. ...

My first impression from this, is that it is possible to call
this as follows:
repeat(None, times = 5)
But that doesn't work either.

The code and/or doc is wrong, you have to use a positional arg
and not a named one. repeat(None, 5) does the right thing.

That wont help much if you would like something like the following:

def fun(f):

arg = Default
try:
arg = Try_Processing( )
except Nothing_To_Proc ess:
pass
f(arg)

Write it like this:

def fun(f):
args = ()
try:
args = (Try_Processing (),)
except Nothing_To_Proc ess:
pass
f(*args)

[Paul Rubin]

Antoon Pardon <ap*****@forel. vub.ac.bewrites :

I think it would become something like:

def myrepeat(*args) :
obj = args[0]
tail = args[1:]
newobj = Process(obj)
newargs = (newobj,) + tail
return itertools.repea t(*newargs)

Too messy. Just write:

def myrepeat(obj, *times):
return itertools.repea t(Process(obj), *times)

[Antoon Pardon]

On 2006-10-03, LaundroMat <La*****@gmail. comwrote:

Suppose I have this function:

def f(var=1):
return var*2

What value do I have to pass to f() if I want it to evaluate var to 1?
I know that f() will return 2, but what if I absolutely want to pass a
value to f()? "None" doesn't seem to work..

Thanks in advance.

I think the only general solution for your problem would be to
define a defaulter function. Something like the following:

Default = object()

def defaulter(f, *args):

while args:
if args[-1] is Default:
args = args[:-1]
else:
break
return f(*args)
The call:

defaulter(f, arg1, arg2, Default, ..., Default)

would then be equivallent to:

f(arg1, arg2)

Or in your case you would call:

defaulter(f, Default)

--
Antoon Pardon

[Antoon Pardon]

On 2006-10-04, Antoon Pardon <ap*****@forel. vub.ac.bewrote:

On 2006-10-03, LaundroMat <La*****@gmail. comwrote:

>Suppose I have this function:

def f(var=1):
return var*2

What value do I have to pass to f() if I want it to evaluate var to 1?
I know that f() will return 2, but what if I absolutely want to pass a
value to f()? "None" doesn't seem to work..

Thanks in advance.

I think the only general solution for your problem would be to
define a defaulter function. Something like the following:

Default = object()

def defaulter(f, *args):

while args:
if args[-1] is Default:
args = args[:-1]
else:
break
return f(*args)
The call:

defaulter(f, arg1, arg2, Default, ..., Default)

would then be equivallent to:

f(arg1, arg2)

Or in your case you would call:

defaulter(f, Default)

A little update, with the functools in python 2.5 you
could turn the above into a decorator. Something like
the following (not tested):

def defaulting(f):
return functools.parti al(defaulter, f)

You could then simply write:

@defaulting
def f(var=1):
return var * 2

And for built in or library functions something like:

from itertools import repeat
repeat = defaulting(repe at)

--
Antoon Pardon

[Georg Brandl]

Paul Rubin wrote:

Antoon Pardon <ap*****@forel. vub.ac.bewrites :

>repeat(objec t[, times])
Make an iterator that returns object over and over again. Runs
indefinitely unless the times argument is specified. ...

My first impression from this, is that it is possible to call
this as follows:
repeat(None, times = 5)
But that doesn't work either.

The code and/or doc is wrong, you have to use a positional arg
and not a named one. repeat(None, 5) does the right thing.

This is an issue in most Python documentation: you're not told
if the described function is implemented in C, and if it is
keyword arg-enabled. The arguments must be given names though,
to be able to document them.

Georg

[Fredrik Lundh]

Georg Brandl wrote:

This is an issue in most Python documentation: you're not told
if the described function is implemented in C, and if it is
keyword arg-enabled. The arguments must be given names though,
to be able to document them.

the general rule is that if the documentation doesn't explicitly say
that something is a keyword argument, it isn't, and shouldn't be treated
as such.

</F>

[Antoon Pardon]

On 2006-10-04, Fredrik Lundh <fr*****@python ware.comwrote:

Georg Brandl wrote:

>This is an issue in most Python documentation: you're not told
if the described function is implemented in C, and if it is
keyword arg-enabled. The arguments must be given names though,
to be able to document them.

the general rule is that if the documentation doesn't explicitly say
that something is a keyword argument, it isn't, and shouldn't be treated
as such.

The first module I looked in to check this, it wasn't true. In the Queue
Module is isn't explicitly written that maxsize is a keyword argument yet
Queue.Queue(max size=9) works just fine.

I then took a look in the Threading module and found that the semaphore
documentation didn't mention anything about keyword arguments but
again they worked fine.

It wouldn't surprise me if this was true for the complete threading
documentation.

--
Antoon Pardon