Wolfram Function Repository
Instantuse addon functions for the Wolfram Language
Function Repository Resource:
Look up a set of keys in order to get deeper parts of an association or list of rules
ResourceFunction["NestedLookup"][assoc,{key_{1},key_{2},…}] looks up the value associated with the keys key_{1},key_{2},… in deeper levels of assoc. 

ResourceFunction["NestedLookup"][assoc,keys, default] gives default if the part specified by keys is not found. 

ResourceFunction["NestedLookup"][{key_{1},key_{2},…}] represents an operator form of ResourceFunction["NestedLookup"] that can be applied to an expression. 
Retrieve the ResourceFunction:
In[1]:= 

Out[1]= 

Look up a nested part of an association:
In[2]:= 

Out[2]= 

When a key is not found, a Missing object is returned:
In[3]:= 

Out[3]= 

When a third argument is provided and the key sequence has a part missing, the default value is returned:
In[4]:= 

Out[4]= 

When the key sequence is present, the default is not evaluated:
In[5]:= 

Out[5]= 

NestedLookup threads over lists of associations:
In[6]:= 

In[7]:= 

Out[7]= 

NestedLookup can be used to look up keys in lists of rules:
In[8]:= 

Out[8]= 

Lists of rules can be mixed with Associations:
In[9]:= 

Out[9]= 

Use the operator form of NestedLookup:
In[10]:= 

Out[10]= 

You can use constructs such as Throw and Return in the third argument of NestedLookup to abort a computation when a required key is missing:
In[11]:= 

In[12]:= 

Out[12]= 

In[13]:= 

Out[13]= 

Applying an Association to a sequence of keys is typically equivalent to using NestedLookup on a list of those keys:
In[14]:= 

Out[14]= 

In[15]:= 

Out[15]= 

Lists are handled differently by NestedLookup, however:
In[16]:= 

Out[16]= 

In[17]:= 

Out[17]= 

Wrap the list in Key to make NestedLookup treat it as a single key:
In[18]:= 

Out[18]= 

When the key to be looked up is a list or an expression with head Key, it must be wrapped in Key to avoid ambiguity:
In[19]:= 

Out[19]= 

In[20]:= 

Out[20]= 

Out[21]= 

In[22]:= 

Out[22]= 

Out[23]= 

When a part is not found, NestedLookup only reports the sequence up to the missing key:
In[24]:= 

Out[24]= 

Wolfram Language 11.3 (March 2018) or above
This work is licensed under a Creative Commons Attribution 4.0 International License