Adding to a dict using a list of key strings as path

Question

I have the following dict: ``` aDict = { "a" : { "b" : { "c1" : {}, "c2" : {}, } } } ``` a second dict: ``` aSecondDict = { "d1" : {}, "d2" : {}, "d3" : {}, } ``` and a "path" tuple: ``` path = ( "a", "b", "c2" ) ``` I now want to add the second dict to the first at the path provided by the tuple: ``` aResultDict = { "a" : { "b" : { "c1" : {}, "c2" : { "d1" : {}, "d2" : {}, "d3" : {}, }, } } } ``` What is the pythonic way of achieving this?

Accepted Answer

You can use `reduce`1 to get the dictionary and `dict.update` to put the new stuff in there:

```
reduce(lambda d,key: d[key],path,aDict).update(aSecondDict)
```

You can even get a little more clever if you want:

```
reduce(dict.__getitem__,path,aDict).update(aSecondDict)
```

I suppose it should be noted that the two approaches are slightly different. The latter forces `aDict` to only contain more dictionaries (or `dict` subclasses) whereas the former allows for anything which has a `__getitem__` method to be in `aDict`. As noted in the comments, you could also use:

```
reduce(dict.get,path,aDict).update(aSecondDict)
```

However, this version will raise an `AttributeError` if you try to traverse a "link" in the path which is non-existent rather than a `KeyError` so I don't like it quite as much. This method also enforces that every value along the path is a `dict` or `dict` subclass.

1`reduce` is a builtin for python2.x. Starting at python2.6 it is also available as `functools.reduce`. Code which wants to be compatible with python3.x should try to use `functools.reduce` as the builtin is removed in python3.x

Adding to a dict using a list of key strings as path

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