clarification
1. The main use case for ChainMap is to provide an efficient way to manage multiple scopes or contexts and handle access prioritization for duplicate keys.
2. This is useful when there are multiple dictionaries storing duplicate keys to access their order.
Find a classic example in the ChainMap documentation that models how Python analyzes variable names in different namespaces.
When Python searches for a name, it searches local, global, and built-in function scopes in turn until it finds the target name.Python scopes are dictionaries that map names to objects.
To simulate Python's internal search chains, chain mapping can be used.
an actual example
>>> import builtins >>> # Shadow input with a global name >>> input = 42 >>> pylookup = ChainMap(locals(), globals(), vars(builtins)) >>> # Retrieve input from the global namespace >>> pylookup["input"] 42 >>> # Remove input from the global namespace >>> del globals()["input"] >>> # Retrieve input from the builtins namespace >>> pylookup["input"] <built-in function input>
Knowledge Point Expansion:
The ChainMap class manages a sequence of dictionaries and searches them in the order of their occurrence to find the value associated with a key.ChainMap provides a good "context" container, so it can be thought of as a stack, with changes occurring as the stack grows, and being discarded as the stack shrinks.
Below, we'll look at the basic rules of its use:
import collections
a = {"a": "A", "c": "c", }
b = {"b": "B", "c": "D", }
col = (a, b)
# Access it like a regular dictionary
print(col["a"])
print(list(()), list(()))
for key, value in ():
print(key, value)
As you can see, in the case of the same key value, only the value of the submap a is available. This also means that ChainMap searches for these submappings in the order in which they are passed to the constructor.
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