What is the purpose of collections.ChainMap?

by

[*]

I like @b4hand’s examples, and indeed I have used in the past ChainMap-like structures (but not ChainMap itself) for the two purposes he mentions: multi-layered configuration overrides, and variable stack/scope emulation.

I’d like to point out two other motivations/advantages/differences of ChainMap, compared to using a dict-update loop, thus only storing the “final” version”:

  1. More information: since a ChainMap structure is “layered”, it supports answering question like: Am I getting the “default” value, or an overridden one? What is the original (“default”) value? At what level did the value get overridden (borrowing @b4hand’s config example: user-config or command-line-overrides)? Using a simple dict, the information needed for answering these questions is already lost.

  2. Speed tradeoff: suppose you have N layers and at most M keys in each, constructing a ChainMap takes O(N) and each lookup O(N) worst-case[*], while construction of a dict using an update-loop takes O(NM) and each lookup O(1). This means that if you construct often and only perform a few lookups each time, or if M is big, ChainMap’s lazy-construction approach works in your favor.

[*] The analysis in (2) assumes dict-access is O(1), when in fact it is O(1) on average, and O(M) worst case. See more details here.

More Related Contents:

  1. Accessing items in an collections.OrderedDict by index
  2. Key-ordered dict in Python
  3. How to select random key with common values?
  4. How to remove [ ] and ” from Python output [duplicate]
  5. How do I sort a list of dictionaries by a value of the dictionary?
  6. Replace values in a pandas series via dictionary efficiently
  7. How to merge multiple dicts with same key or different key?
  8. What would a “frozen dict” be?
  9. Why is dictionary ordering non-deterministic?
  10. Error: ” ‘dict’ object has no attribute ‘iteritems’ “
  11. count the number of occurrences of a certain value in a dictionary in python?
  12. TypeError: ‘dict_keys’ object does not support indexing
  13. Is there anything faster than dict()?
  14. Using a Python Dictionary as a Key (Non-nested)
  15. Why is a class __dict__ a mappingproxy?
  16. Extract a subset of key-value pairs from dictionary?
  17. How to make a dictionary that returns key for keys missing from the dictionary instead of raising KeyError?
  18. Are there any reasons not to use an OrderedDict?
  19. How to get count dict of items but maintain the order in which they appear?
  20. How do I return dictionary keys as a list in Python?
  21. How to reverse a dictionary (whose values are lists) in Python? [duplicate]
  22. Changing one dict changes all dicts in a list of dicts [duplicate]
  23. Python dictionary replace values
  24. enumerate() for dictionary in Python
  25. Appending to list in Python dictionary [duplicate]
  26. Adding counters deletes keys
  27. Python – How to convert only numbers in a mixed list into float?
  28. How to change all the dictionary keys in a for loop with d.items()?
  29. How to sort a list of dictionaries by a value of the dictionary in Python?
  30. How to add an element to the beginning of an OrderedDict?

Leave a Comment