Missing Something Simple
Hi, I have a list of variables, which I am iterating over. I need to set the value of each variable. My code looks like: varList = [ varOne, varTwo, varThree, varFour ] for indivVar in varList: indivVar = returnVarFromFunction() However, none of the variables in the list are being set. I thought of using setattr, but this code sits in a function, and not class, so I'm unsure what the object would be. I'm hoping someone can point me in the right direction. John -- http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
Hi, I have a list of variables, which I am iterating over. I need to set the value of each variable. My code looks like: varList = [ varOne, varTwo, varThree, varFour ] for indivVar in varList: indivVar = returnVarFromFunction() However, none of the variables in the list are being set. You only change the value of the local variable in the body of the for loop. it has no effect on the list. you could do e.g. varList = [vorOne,varTwo,varThree,varFour] for i in len(varList) : varList[i] = returnVarFromFunction() However, as in this example the former list values are not used anyway, you could just write: varList = [ returnVarFromFunction for i varList ] cheers, - harold - -- Tages Arbeit, abends Gäste, saure Wochen, frohe Feste! -- Johann Wolfgang v. Goethe -- http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
harold fellermann wrote: Hi, I have a list of variables, which I am iterating over. I need to set the value of each variable. My code looks like: varList = [ varOne, varTwo, varThree, varFour ] for indivVar in varList: indivVar = returnVarFromFunction() However, none of the variables in the list are being set. You only change the value of the local variable in the body of the for loop. it has no effect on the list. you could do e.g. varList = [vorOne,varTwo,varThree,varFour] for i in len(varList) : varList[i] = returnVarFromFunction() However, as in this example the former list values are not used anyway, you could just write: varList = [ returnVarFromFunction for i varList ] cheers, - harold - -- Tages Arbeit, abends Gäste, saure Wochen, frohe Feste! -- Johann Wolfgang v. Goethe The problem I have, is the variables are referenced elsewhere. They have been declared before being used in the list. Basically, I'm after the Python way of using deferencing. J -- http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
have been declared before being used in the list. Basically, I'm after the Python way of using deferencing. OK, that should say dereferencing. J -- http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
Am Tue, 12 Jul 2005 14:44:00 +0100 schrieb John Abel: Hi, I have a list of variables, which I am iterating over. I need to set the value of each variable. My code looks like: varList = [ varOne, varTwo, varThree, varFour ] for indivVar in varList: indivVar = returnVarFromFunction() However, none of the variables in the list are being set. I thought of using setattr, but this code sits in a function, and not class, so I'm unsure what the object would be. Hi, indivVar is a *reference* to a value. You only change the reference, not the value. Maybe this code helps you: a=a b=b c=c varList = [a, b, c] for x in varList: x = foo # nothing changed print varList # -- [a, b, c] for i in range(len(varList)): varList[i]=foo # List was changed print varList # -- [foo, foo, foo] HTH, Thomas -- Thomas Güttler, http://www.thomas-guettler.de/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
Hello John,
John Abel wrote:
harold fellermann wrote:
Hi,
I have a list of variables, which I am iterating over. I need to set
the value of each variable. My code looks like:
varList = [ varOne, varTwo, varThree, varFour ]
for indivVar in varList:
indivVar = returnVarFromFunction()
However, none of the variables in the list are being set.
You only change the value of the local variable in the body
of the for loop. it has no effect on the list. you could do e.g.
varList = [vorOne,varTwo,varThree,varFour]
for i in len(varList) :
varList[i] = returnVarFromFunction()
However, as in this example the former list values are not used anyway,
you could just write:
varList = [ returnVarFromFunction for i varList ]
cheers,
- harold -
--
Tages Arbeit, abends Gäste,
saure Wochen, frohe Feste!
-- Johann Wolfgang v. Goethe
The problem I have, is the variables are referenced elsewhere. They
have been declared before being used in the list. Basically, I'm after
the Python way of using deferencing.
The problem you have is that you don't understand the way that Python
references objects.
All Python names (aka variables) are references. You can rebind a name
to *any* object, but you can only change *some* objects. These are
called the mutable datatypes. The ones you can't changed are called
immutable types.
This is a common Python gotcha - but it's an integral part of the way
Python works - not a wart.
Your problem (I think) is that you have something like :
myVar = 'hello'
another_name = myVar
another_name = 'goodbye'
print myVar
'hello'
but you expected 'goodbye'.
What you have done in the first line is created a new - a string with
the contents 'hello' - and bound the name
In the second line you bind another name to the *same* object. (You
*don't* bind the second name to the first name, but to the object it
references).
In the third line you create a new object and *rebind* the second name.
You haven't chanegd the underlying object. In Python the string is
immutable. This means it's hashable and can be used as a dictionary
key.
If you want to maintain a reference to a *location* then use a mutable
datatype. Instead of a list use a dictionary, keyed by name (as one
example).
e.g. a_dict = {'name1': object1, 'name2': object2}
Even if you change the contents of the dictionaries, the names will
still point to what you expect. (And you can still iterate over a
dictionary).
Before you get much further in Python you'll need a clearer
understanding of the difference between it's objects and names.
Best Regards,
Fuzzy
http://www.voidspace.org.uk/python
J
--
http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
I have a list of variables, which I am iterating over. I need to set the value of each variable. My code looks like: varList = [ varOne, varTwo, varThree, varFour ] for indivVar in varList: indivVar = returnVarFromFunction() However, none of the variables in the list are being set. You only change the value of the local variable in the body of the for loop. it has no effect on the list. you could do e.g. varList = [vorOne,varTwo,varThree,varFour] for i in len(varList) : varList[i] = returnVarFromFunction() However, as in this example the former list values are not used anyway, you could just write: varList = [ returnVarFromFunction for i varList ] The problem I have, is the variables are referenced elsewhere. They have been declared before being used in the list. Basically, I'm after the Python way of using deferencing. so, if I understand you right, what you want to have is a list of mutable objects, whose value you can change without changing the objects' references. class Proxy : def __init__(self,val) : self.set(val) def set(self,val) : self.val = val def get(self) : return self.val a = Proxy(1) b = Proxy(2) c = Proxy(3) varList = [a,b,c] for i in varList : i.set(returnVarFromFunction()) print a,b,c prints whatever returnVarFromFunction has returned. n.b.: instead of the Proxy class, you can use any other mutable objects, e.g. lists. - harold - -- All unsere Erfindungen sind nichts als verbesserte Mittel zu einem nicht verbesserten Zweck. -- H.D. Thoreau -- http://mail.python.org/mailman/listinfo/python-list
Re: Missing Something Simple
harold fellermann wrote: so, if I understand you right, what you want to have is a list of mutable objects, whose value you can change without changing the objects' references. Yes! class Proxy : def __init__(self,val) : self.set(val) def set(self,val) : self.val = val def get(self) : return self.val a = Proxy(1) b = Proxy(2) c = Proxy(3) varList = [a,b,c] for i in varList : i.set(returnVarFromFunction()) print a,b,c prints whatever returnVarFromFunction has returned. n.b.: instead of the Proxy class, you can use any other mutable objects, e.g. lists. - harold - -- All unsere Erfindungen sind nichts als verbesserte Mittel zu einem nicht verbesserten Zweck. -- H.D. Thoreau That does it. Thank you! J -- http://mail.python.org/mailman/listinfo/python-list
