list indices must be integers python nested dictionaries

Question

In python 3, I need a function to dynamically return a value from a nested key.

nesteddict = {'a':'a1','b':'b1','c':{'cn':'cn1'}}
print(nesteddict['c']['cn']) #gives cn1

def nestedvalueget(keys):
    print(nesteddict[keys])

nestedvalueget(['n']['cn'])

How should nestedvalueget be written?

I'm not sure the title is properly phrased, but I'm not sure how else to best describe this.

Answer 1

If you want to traverse dictionaries, use a loop:

def nestedvalueget(*keys):
    ob = nesteddict
    for key in keys:
        ob = ob[key]
    return ob

or use functools.reduce() :

from functools import reduce
from operator import getitem

def nestedvalueget(*keys):
    return reduce(getitem, keys, nesteddict)

then use either version as:

nestedvalueget('c', 'cn')

Note that either version takes a variable number of arguments to let you pas 0 or more keys as positional arguments.

Demos:

>>> nesteddict = {'a':'a1','b':'b1','c':{'cn':'cn1'}}
>>> def nestedvalueget(*keys):
...     ob = nesteddict
...     for key in keys:
...         ob = ob[key]
...     return ob
... 
>>> nestedvalueget('c', 'cn')
'cn1'
>>> from functools import reduce
>>> from operator import getitem
>>> def nestedvalueget(*keys):
...     return reduce(getitem, keys, nesteddict)
... 
>>> nestedvalueget('c', 'cn')
'cn1'

And to clarify your error message: You passed the expression ['n']['cn'] to your function call, which defines a list with one element ( ['n'] ), which you then try to index with 'cn' , a string. List indices can only be integers:

>>> ['n']['cn']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers, not str
>>> ['n'][0]
'n'