Python: access variable from string

Question

This is probably something very basic and simple, and I'm probably just googling for the wrong terms, but hopefully someone here can help me. (I'm still a beginner to programming, which is probably obvious from this question.)

I'm looking for a way to access variables from strings.

Like this:

A1 = {}
B1 = {}
C1 = {}

mylist = ["A","B","C"]

for blurb in mylist:
    blurb1.foo()

Right now I'm using if/elif-constructions in such cases:

if blurb == "A":
    A1.foo()
elif blurb == "B":
    B1.foo()
elif blurb == "C":
    C1.foo()

That works, but surely there's a more elegant way of doing it?

Thanks for any help! Lastalda

---

Edit2: trying to clarify again (sorry for not being very coherent before, it's hard for me to get this across):

I would like to create and access objects from strings.

So if i have a function that returns a string, I want to create e.g. a list using this string as the list name, and then do stuff with that list.

x = foo() # foo returns a string, e.g. "A"
# create list named whatever the value of x is
# in this case: 
A = []

Isn't there anything more elegant than preparing lists for all possible outcomes of x and using long elif constructions or dictionaries? Do I have to use eval() in that case? (I'm hesitant to use eval(), as it's considered dangerous.)

I hope it's finally clear now. Any help still appreciated!

Answer 1

I think you want something like

lookup = {"A": A1, "B": B1, "C": C1}
obj = lookup.get(blurb,None)
if obj:
   obj.foo()

This is usually suggested when someone wants to do something with a switch statement (from c/java/etc.) Note that the lookup.get (instead of just lookup[blurb] handles the case where blurb is something other than one of the values you defined it for... you could also wrap that in a try/except block.

---

Updated answer based on your revised question, which builds on what Joachim Pileborg had said in another answer.

There are probably better/cleaner ways to do this, but if you really want to set/get via strings to manipulate global objects, you can do that. One way would be: To create a new object named "myvariable" (or to set "myvariable" to a new value)

joe = "myvariable"
globals().update([[joe,"myvalue"]])

To get the object "myvariable" is assigned to:

globals().get(joe,None) 

(or as mentioned in another answer, you could use try/except with direct hash instead of using get)

So for later example, you could do something like:

for myobject in mylist:
    # add myobject to list called locus
    # e.g. if locus == "A", add to list "A", if locus == "B" add to list B etc.
    obj = globals().get(myobject.locus,None) # get this list object
    if not obj: # if it doesn't exist, create an empty list named myobject.locus
        obj = []
        globals().update([[myobject.locus,obj]]
    obj.append(myobject) #append my object to list called locus

Answer 2

You could get the module object, and use getattr to get variables/functions from a string:

import sys

A = {}
B = {}
C = {}

modname = globals()['__name__']
modobj  = sys.modules[modname]

mylist = ["A","B","C"]

for name in mylist:
    print '%s : %r' % (name, getattr(modobj, name))

The above program will print:

A : {}
B : {}
C : {}

However, it has nothing to do with "wildcards" that you reference in your question, but you don't show any searching for "wildcards".

Answer 3

use eval().

example:

A1 = []
B1 = []
C1 = []

mylist = ["A","B","C"]

for blurb in mylist:
    eval("%s1.append(blurb)" % blurb)

print A1,B1,C1
>>> 
['A'] ['B'] ['C']

and for your code it can be:

for blurb in mylist:
    eval("%s1.foo()" % blurb)

Answer 4

I looks like you simply want a dictionary and to look it up rather than run a bunch of checks.

dict_look_up = {'A' : A1, 'B' : B1, 'C' : C1}
try:
    dict_look_up[blurb].foo()
except KeyError:
    #whatever on item not found

try/except blocks are incredibly important in Python (better to ask for forgiveness than permission), so if you're learning I recommend using them from the get go.