Changing variable names with Python for loops

Question

I was just wondering if anyone knew of a way to change variable names based off of a for loop for something like this:

for i in range(3)
     group+i=self.getGroup(selected, header+i)

so that the names of the variables change to accomodate the data. Thanks!

~Sam

Solution 1

You probably want a dict instead of separate variables. For example

d = {}
for i in range(3):
    d["group" + str(i)] = self.getGroup(selected, header+i)

If you insist on actually modifying local variables, you could use the locals function:

for i in range(3):
    locals()["group"+str(i)] = self.getGroup(selected, header+i)

On the other hand, if what you actually want is to modify instance variables of the class you're in, then you can use the setattr function

for i in group(3):
    setattr(self, "group"+str(i), self.getGroup(selected, header+i)

And of course, I'm assuming with all of these examples that you don't just want a list:

groups = [self.getGroup(i,header+i) for i in range(3)]

Solution 2

Use a list.

groups = [0]*3
for i in xrange(3):
    groups[i] = self.getGroup(selected, header + i)

or more "Pythonically":

groups = [self.getGroup(selected, header + i) for i in xrange(3)]

For what it's worth, you could try to create variables the "wrong" way, i.e. by modifying the dictionary which holds their values:

l = locals()
for i in xrange(3):
    l['group' + str(i)] = self.getGroup(selected, header + i)

but that's really bad form, and possibly not even guaranteed to work.

Solution 3

Definitely should use a dict using the "group" + str(i) key as described in the accepted solution but I wanted to share a solution using exec. Its a way to parse strings into commands & execute them dynamically. It would allow to create these scalar variable names as per your requirement instead of using a dict. This might help in regards what not to do, and just because you can doesn't mean you should. Its a good solution only if using scalar variables is a hard requirement:

l = locals()
for i in xrange(3):
    exec("group" + str(i) + "= self.getGroup(selected, header + i)")

Another example where this could work using a Django model example. The exec alternative solution is commented out and the better way of handling such a case using the dict attribute makes more sense:

Class A(models.Model):

    ....

    def __getitem__(self, item):  # a.__getitem__('id')
        #exec("attrb = self." + item)
        #return attrb
        return self.__dict__[item]

It might make more sense to extend from a dictionary in the first place to get setattr and getattr functions.

A situation which involves parsing, for example generating & executing python commands dynamically, exec is what you want :) More on exec here.

Solution 4

It looks like you want to use a list instead:

group=[]
for i in range(3):
     group[i]=self.getGroup(selected, header+i)

View more solutions

Comments

[user1066101]

Do you mean "Dictionary" or "List"? What's an Array?

[Greg Hewgill]

I mean List, thanks. I've called ordered collections of items "arrays" for a very long time, and have never used PHP where they usurped the term to mean "associative array".

[tgray]

Instead of using self.__dict__[etc] you'll want to use setattr(self, etc), and I'm sure you meant to say 'list' instead of 'array'. Good use of string formatting though.

[Emmanuel Manolo Siame]

what if the range is unknown, how can I approach this problem?

[DC007744]

@EmmanuelManoloSiame you can use the len() method in Python. Read more about the len() method here.