Real Python: Python's ChainMap: Manage Multiple Contexts Effectively

Sometimes when you’re working with several different dictionaries, you need to group and manage them as a single one. In other situations, you can have multiple dictionaries representing different scopes or contexts and need to handle them as a single dictionary that allows you to access the underlying data following a given order or priority. In those cases, you can take advantage of Python’s ChainMap from the collections module.

ChainMap groups multiple dictionaries and mappings in a single, updatable view with dictionary-like behavior. Additionally, ChainMap provides features that allow you to efficiently manage various dictionaries, define key lookup priorities, and more.

In this tutorial, you’ll learn how to:

• Create ChainMap instances in your Python programs
• Explore the differences between ChainMap and dict
• Use ChainMap to work with several dictionaries as one
• Manage key lookup priorities with ChainMap

To get the most out of this tutorial, you should know the basics of working with dictionaries and lists in Python.

By the end of the journey, you’ll find a few practical examples that will help you better understand the most relevant features and use cases of ChainMap.

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Getting Started With Python’s ChainMap

Python’s ChainMap was added to collections in Python 3.3 as a handy tool for managing multiple scopes and contexts. This class allows you to group several dictionaries and other mappings together to make them logically appear and behave as one. It creates a single updatable view that works similar to a regular dictionary but with some internal differences.

ChainMap doesn’t merge its mappings together. Instead, it keeps them in an internal list of mappings. Then ChainMap reimplements common dictionary operations on top of that list. Since the internal list holds references to the original input mapping, any changes in those mappings affect the ChainMap object as a whole.

Storing the input mappings in a list allows you to have duplicate keys in a given chain map. If you perform a key lookup, then ChainMap searches the list of mappings until it finds the first occurrence of the target key. If the key is missing, then you get a KeyError as usual.

Storing the mappings in a list truly shines when you need to manage nested scopes, where each mapping represents a specific scope or context.

To better understand what scopes and contexts are about, think about how Python resolves names. When Python looks for a name, it searches in locals(), globals(), and finally builtins until it finds the first occurrence of the target name. If the name doesn’t exist, then you get a NameError. Dealing with scopes and contexts is the most common kind of problem you can solve with ChainMap.

When you’re working with ChainMap, you can chain several dictionaries with keys that are either disjoint or intersecting.

In the first case, ChainMap allows you to treat all your dictionaries as one. So, you can access the key-value pairs as if you were working with a single dictionary. In the second case, besides managing your dictionaries as one, you can also take advantage of the internal list of mappings to define some sort of access priority for repeated keys across your dictionaries. That’s why ChainMap objects are great for handling multiple contexts.

A curious behavior of ChainMap is that mutations, such as updating, adding, deleting, clearing, and popping keys, act only on the first mapping in the internal list of mappings. Here’s a summary of the main features of ChainMap:

• Builds an updatable view from several input mappings
• Provides almost the same interface as a dictionary, but with some extra features
• Doesn’t merge the input mappings but instead keeps them in an internal public list
• Sees external changes in the input mappings
• Can contain repeated keys with different values
• Searches keys sequentially through the internal list of mappings
• Throws a KeyError when a key is missing after searching the entire list of mappings
• Performs mutations only on the first mapping in the internal list

In this tutorial, you’ll learn a lot more about all these cool features of ChainMap. The following section will guide you through how to create new instances of ChainMap in your code.

Instantiating ChainMap

To create ChainMap in your Python code, you first need to import the class from collections and then call it as usual. The class initializer can take zero or more mappings as arguments. With no arguments, it initializes a chain map with an empty dictionary inside:

>>>

>>> from collections import ChainMap
>>> from collections import OrderedDict, defaultdict

>>> # Use no arguments
>>> ChainMap()
ChainMap({})

>>> # Use regular dictionaries
>>> numbers = {"one": 1, "two": 2}
>>> letters = {"a": "A", "b": "B"}

>>> ChainMap(numbers, letters)
ChainMap({'one': 1, 'two': 2}, {'a': 'A', 'b': 'B'})

>>> ChainMap(numbers, {"a": "A", "b": "B"})
ChainMap({'one': 1, 'two': 2}, {'a': 'A', 'b': 'B'})

>>> # Use other mappings
>>> numbers = OrderedDict(one=1, two=2)
>>> letters = defaultdict(str, {"a": "A", "b": "B"})
>>> ChainMap(numbers, letters)
ChainMap(
    OrderedDict([('one', 1), ('two', 2)]),
    defaultdict(<class 'str'>, {'a': 'A', 'b': 'B'})
)

Here, you create several ChainMap objects using different combinations of mappings. In each case, ChainMap returns a single dictionary-like view of all the input mappings. Note that you can use any type of mapping, such as OrderedDict and defaultdict.

You can also create ChainMap objects using the class method .fromkeys(). This method can take an iterable of keys and an optional default value for all the keys:

>>>

>>> from collections import ChainMap

>>> ChainMap.fromkeys(["one", "two","three"])
ChainMap({'one': None, 'two': None, 'three': None})

>>> ChainMap.fromkeys(["one", "two","three"], 0)
ChainMap({'one': 0, 'two': 0, 'three': 0})

If you call .fromkeys() on ChainMap with an iterable of keys as an argument, then you get a chain map with a single dictionary. The keys come from the input iterable, and the values default to None. Optionally, you can pass a second argument to .fromkeys() to provide a sensible default value for every key.

Read the full article at https://realpython.com/python-chainmap/ »

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