using a list of strings to traverse a dictionary python

Question

I have a dictionary in python that represents a directory structure, so something like

dict = {
    "file1.java": {
          "method1": [],
          "method2": []
     },
     "dir1": {
          "file2.java":{
           .
           .
           .
           },
           "dir2": {
                    "file3.java": {
                           "method3": ["hello", "hi"],
                           "method4": []
                    "file4.java": {
            }
     }

}

I also have have a list of strings like

list = ["dir1", "dir2", "file3.java", "method3"]

Is there an easy way to traverse the dictionary to access the data in method3 using the list So I would like to be able to call

dict[list]

and have it do something like

 dict["dir1"]["dir2"]["file3.java"]["method3"]

and return something like

["hello", "hi"]

but I have no idea how long the list of strings will be so the list

list = ["file1.java","method1"]

also needs to work

You can't do that in a vanilla dictionary, but you could either: 1. Create a custom `dict`-like class with a `__getitem__`/`__setitem__` implementation that supports it; or 2. Write a function that takes the dictionary and list of keys. Either way, though, this isn't a code-writing service. — jonrsharpe, Jul 07 '15 at 16:53
`dict` si a keyword - please do not use it as a variable name! — Nir Alfasi, Jul 07 '15 at 16:53
Probable duplicate: http://stackoverflow.com/q/14692690/3001761 — jonrsharpe, Jul 07 '15 at 16:54
@alfasin not a keyword, strictly speaking, but it's certainly wisest not to shadow it! — jonrsharpe, Jul 07 '15 at 16:56
@jonrsharpe how would I write a function that would accomplish it though. I guess i realized it wouldn't be in vanilla python, but I am unsure how to do it for myself — pythonHelpRequired, Jul 07 '15 at 16:56
**This isn't a code-writing service** - what have you tried, and what exactly is the problem with it? — jonrsharpe, Jul 07 '15 at 16:57
@jonrsharpe thanks for the correction! what is the right term then ? — Nir Alfasi, Jul 07 '15 at 16:59
@alfasin I usually describe it as *"the name of a built-in [function/type/whatever they're shadowing]"*. — jonrsharpe, Jul 07 '15 at 17:00
@jonrsharpe I think I see the distinction, since this is a (builtin) function and not a "keyword" such as `if` `while` etc, is that correct ? — Nir Alfasi, Jul 07 '15 at 17:01
@alfasin yes, exactly - the keywords (see e.g. https://docs.python.org/2/reference/lexical_analysis.html#keywords, https://docs.python.org/2/library/keyword.html) can't be assigned to at all, hence can't be shadowed. — jonrsharpe, Jul 07 '15 at 17:02

score 1 · Answer 1 · answered Jul 07 '15 at 17:13

You can use the reduce built-in function.

db = {
    "file1.java": {
          "method1": [],
          "method2": []
     },
     "dir1": {
           "dir2": {
                    "file3.java": {
                           "method3": ["hello", "hi"],
                           "method4": []
                    },
                    "file4.java": {
                    }
            }
     }

}

def lookup(db, path):
    return reduce(dict.get, path, db)

print lookup( db, ["dir1", "dir2", "file3.java", "method3"] )


print lookup( db, ["file1.java","method1"] )

score 0 · Answer 2 · answered Jul 07 '15 at 17:11

You can take advantage of the fact that dicts and arrays are reference types to do something like this...

current_entry = dict
for item in list:
    current_entry = current_entry[item]

# The element you want is now in current_entry

Of course, add appropriate error checking to make sure you don't try and select a dict element that doesn't exist.

using a list of strings to traverse a dictionary python

2 Answers2