Blog
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Merging Python dictionaries with |: Right side wins conflicts
Merge two Python dicts with | which returns a new dict. Conflicts? The right-side dict gets priority: d1 | d2 # {‘a’: 10, ‘b’: 100, ‘c’: 200, ‘d’: 300}d2 | d1 # {‘b’: 20, ‘c’: 30, ‘d’: 300, ‘a’: 10}
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Modifying a Python dict during iteration raises RuntimeError
Iterating over a Python dict? Modifying existing values is fine, but adding or removing keys during iteration raises RuntimeError:
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Python dict iteration: Using items to get keys and values
Want to iterate over a Python dict? The “items” method is your friend, returning a (key, value) tuple with each iteration: Remember: Insertion order determines iteration order.
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Python dictionary equality: Insertion order doesn’t matter
Dictionaries in Python remember their insertion order, but equality ignores the ordering:
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Python dict of lists: Using defaultdict from collections
Want to create a dict of lists in Python? You can use defaultdict:
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Python dicts of lists: Grouping values with setdefault
Want to create a dict of lists in Python? Use setdefault:
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Python dict.setdefault: Assign only when the key is missing
Want to assign to a Python dict, but only if the key doesn’t yet exist? Use setdefault:
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Python dictionaries: Avoiding KeyError with dict.get
Retrieve a value from a Python dict with the key: Request a key that doesn’t exist: Avoid that with dict.get, what I call “forgiving []”
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Python dict key lookup: Use in instead of d.keys()
Want to check if a Python dict contains a key? Use “in” on the dict: ‘a’ in d # True‘x’ in d # False Searching in d.keys() is far slower: ‘a’ in d # 10 ns‘a’ in d.keys() # 29 ns (!)
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Python dict creation speed: Literal vs dict vs comprehension
Want to create a Python dict? You have options; which is fastest? – Literal, with {‘a’:10, ‘b’:20}: 37.8 ns– Invoke dict(a=10, b=20): 58.5 ns– Invoke dict on a list of tuples, dict([(‘a’, 10), (‘b’, 20)]): 103 ns– Comprehension, {c : ord(c) for c in ‘ab’}: 78 ns