Python Dict Comprehension
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new_dict = {key_expr: value_expr for item in iterable if condition}Basic Dict Comprehension
Dictionary comprehensions build a new dict in one expression using {key_expr: value_expr for item in iterable}, which is the dict equivalent of a list comprehension and is typically faster and more readable than looping with an empty dict and repeated assignment.
Example: Basic Dict Comprehension
nums = [1, 2, 3]
squares = {n: n * n for n in nums} # key is n, value is its square
print(squares)Filtering with If-Conditions
Adding 'if condition' at the end filters which source items get turned into entries -- {k: v for k, v in pairs if v > 0} keeps only the pairs whose value is positive, discarding the rest entirely rather than transforming them.
Example: Filtering with If-Conditions
pairs = {"a": 1, "b": -2, "c": 3}
positive = {k: v for k, v in pairs.items() if v > 0} # if filters out non-positive values
print(positive)Transforming Keys and Values
You can compute both the key and the value from the same source item, letting you reshape data as you go -- for example {name.lower(): len(name) for name in names} both normalizes the key and derives a new value in a single pass.
Example: Transforming Keys and Values
names = ["Alice", "Bob"]
lengths = {name.lower(): len(name) for name in names} # key is lowercased, value is the length
print(lengths)Creating Dict from Two Lists
zip() pairs up two same-length lists element by element, and wrapping that in a comprehension -- {k: v for k, v in zip(keys, values)} -- turns two parallel lists into a single dictionary without a manual loop.
Example: Creating Dict from Two Lists
keys = ["a", "b", "c"]
values = [1, 2, 3]
combined = {k: v for k, v in zip(keys, values)} # zip pairs the two lists element by element
print(combined)Comprehension with If-Else Conditions
Placing a ternary if-else before the for clause lets every key get a value based on a condition rather than being excluded -- {k: pass if v >= 50 else fail for k, v in scores.items()} keeps every key while varying its mapped value.
Example: Comprehension with If-Else Conditions
scores = {"Alex": 80, "Sam": 40}
results = {k: "pass" if v >= 50 else "fail" for k, v in scores.items()} # every key is kept, value depends on the condition
print(results)- Forgetting the colon, as in
{k v for k, v in pairs}, which is aSyntaxError; a dict comprehension needskey: value. - Building duplicate keys and expecting all values to be kept, when later entries overwrite earlier ones with the same key.
- Writing
{}around a plain expression such as{x for x in items}, which creates a set, not a dictionary.
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