Traversing objects in python

Question

How can I write this code without repeating myself indefinitely?

fields = row.split('__')

if len(fields) == 1:
    foo = getattr(bundle.obj, fields[0])
elif len(fields) == 2:
    foo = getattr(getattr(bundle.obj, fields[0]), fields[1])
elif len(fields) == 3:
    foo = getattr(getattr(getattr(bundle.obj,
                            fields[0]), fields[1]), fields[2])
# etc ..

Martijn Pieters · Accepted Answer · 2013-07-03 22:55:57Z

10

Use reduce():

foo = reduce(getattr, fields, bundle.obj)

or a simple loop:

foo = bundle.obj
for field in fields:
    foo = getattr(foo, field)

answered Jul 3, 2013 at 22:55

Martijn Pieters

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6 Comments

Jason Sperske Over a year ago

I feel like every day I learn about another built-in function that is incredibly useful. That Guido is one smart benevolent dictator :)

abarnert Over a year ago

@JasonSperske: Actually, Guido can't remember who convinced him to add reduce in the early days, but he's wanted to get rid of it ever since. And many of the other core devs agree with him. They hate it with a passion, and in the 3.0 transition, were just barely persuaded to move it to functools and just stop talking about it instead of removing it entirely. (Personally, I think there are some cases where reduce makes things more readable—rare, but this is definitely one of them.)

torek Over a year ago

@abarnert: No doubt it was one of those obnoxious Lisp people. :D (I'd say "one of us" except that I never wrote much Lisp, even when I was hanging out with the AI grad students....) Hasn't there been a lot of grumbling about map as well?

torek Over a year ago

@JasonSperske: I think a lot of it is a matter of taste and a knowledge of history. Both map and reduce came out of various functional languages, and lots of bits of Python are stolen I mean borrowed from other languages as well. Getting the right mix of ideas with workable syntax and so on, well, that's where "taste" comes in. Python has it, perl ... not so much. :D

abarnert Over a year ago

@torek: A lot less. Guido did originally suggest getting rid of map and filter when he added comprehensions, but since various people have started using map to contrast with reduce (as in "not all functional tools are as useless as reduce—look at map, which everyone loves as a builtin") he's come around to their way of thinking.

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