Inverting a dictionary in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to invert a dictionary?

Inverting a dictionary means swapping its keys and values. The original keys become values, and the original values become keys.

beginner

How can you invert a dictionary in Python using a dictionary comprehension?

You can invert a dictionary by creating a new dictionary where each key-value pair is swapped:

inverted = {v: k for k, v in original.items()}

intermediate

What problem can occur if the original dictionary has duplicate values when inverting?

If the original dictionary has duplicate values, inverting it will lose data because keys in a dictionary must be unique. Only the last key for a duplicated value will be kept.

beginner

Show a Python code example that inverts a dictionary and explain the output.

original = {'a': 1, 'b': 2, 'c': 3}

Code:

inverted = {v: k for k, v in original.items()}
print(inverted)

Output:

{1: 'a', 2: 'b', 3: 'c'}

Explanation: Each key-value pair is swapped, so 1 becomes a key with value 'a', and so on.

intermediate

How can you invert a dictionary that has duplicate values without losing data?

You can invert it by making the values keys that map to a list of original keys:

inverted = {}
for k, v in original.items():
    inverted.setdefault(v, []).append(k)

What happens if you invert a dictionary with duplicate values using a simple comprehension?

A It will raise an error

B Some keys will be lost because dictionary keys must be unique

C The dictionary will keep all keys and values as is

D The dictionary will become empty

When inverting, duplicate values become keys, but keys must be unique, so only the last key-value pair is kept.

Which Python method helps to iterate over key-value pairs in a dictionary?

A items()

B keys()

C values()

D get()

The items() method returns key-value pairs, which is useful for inverting dictionaries.

What is the output of this code?

original = {'x': 10, 'y': 20}
inverted = {v: k for k, v in original.items()}
print(inverted)

A {'x': 10, 'y': 20}

B {'10': 'x', '20': 'y'}

C {10: 'x', 20: 'y'}

D Error

The dictionary is inverted by swapping keys and values.

How can you invert a dictionary and keep all original keys if values are duplicated?

A Use keys() method only

B Use a simple comprehension

C Ignore duplicates

D Map each value to a list of keys

Mapping values to lists of keys preserves all keys even if values repeat.

What type of Python object is created when you invert a dictionary?

A Another dictionary

B A list

C A set

D A tuple

Inverting a dictionary produces a new dictionary with keys and values swapped.

Explain how to invert a dictionary in Python and what issues might arise with duplicate values.

Think about how keys must be unique in dictionaries.

Write a Python code snippet to invert a dictionary that may have duplicate values, preserving all original keys.

Use a dictionary where each key maps to a list.