Creating dictionary from two sequences in Python

Introduction

We use this to pair items from two lists or sequences into a dictionary, making it easy to look up values by keys.

You have a list of names and a list of phone numbers and want to link each name to its phone number.

You want to combine two lists, like countries and their capitals, into a dictionary for quick access.

You have two sequences of related data and want to organize them as key-value pairs.

You want to convert two separate lists into one dictionary for easier data handling.

Syntax

Python

dictionary = dict(zip(sequence1, sequence2))

The zip() function pairs items from both sequences one by one.

dict() converts these pairs into a dictionary.

Examples

Creates a dictionary {'a': 1, 'b': 2, 'c': 3} by pairing keys and values.

Python

keys = ['a', 'b', 'c']
values = [1, 2, 3]
d = dict(zip(keys, values))
print(d)

Pairs fruit names with their colors in a dictionary.

Python

fruits = ['apple', 'banana']
colors = ['red', 'yellow']
fruit_colors = dict(zip(fruits, colors))
print(fruit_colors)

Maps numbers to their squares using two lists.

Python

numbers = [1, 2, 3]
squares = [1, 4, 9]
square_dict = dict(zip(numbers, squares))
print(square_dict)

Sample Program

This program creates a dictionary linking each person's name to their age.

Python

names = ['John', 'Jane', 'Doe']
ages = [25, 30, 22]
people_ages = dict(zip(names, ages))
print(people_ages)

Important Notes

If the sequences have different lengths, zip stops at the shortest one.

Keys in a dictionary must be unique; if duplicates exist in the first sequence, later values overwrite earlier ones.

Summary

Use zip() to pair items from two sequences.

Convert the pairs to a dictionary with dict().

This method helps organize related data simply and clearly.

Practice

1. What does the Python function zip() do when used with two sequences?

easy

A. Sorts both sequences in ascending order

B. Adds elements of both sequences together

C. Removes duplicate elements from both sequences

D. Pairs elements from both sequences into tuples

Solution

1. Step 1: Understand the purpose of zip()

   The zip() function pairs elements from two sequences by their positions, creating tuples.

2. Step 2: Recognize the output format

   Each tuple contains one element from each sequence, matched by index.

3. Final Answer:

   Pairs elements from both sequences into tuples -> Option D

4. Quick Check:

   zip() pairs elements = C [OK]

Hint: Remember zip pairs items by position from sequences [OK]

Common Mistakes:

• Thinking zip adds or subtracts elements
• Assuming zip sorts sequences
• Believing zip removes duplicates

2. Which of the following is the correct syntax to create a dictionary from two lists keys and values?

easy

A. dict(zip(keys, values))

B. dict(keys, values)

C. zip(dict(keys), dict(values))

D. dict(keys + values)

Solution

1. Step 1: Understand dict() and zip() usage

   The dict() function can convert an iterable of key-value pairs into a dictionary. zip(keys, values) creates these pairs.

2. Step 2: Check each option's correctness

   dict(zip(keys, values)) correctly uses dict(zip(keys, values)). Others misuse dict() or zip() syntax.

3. Final Answer:

   dict(zip(keys, values)) -> Option A

4. Quick Check:

   dict(zip(keys, values)) = B [OK]

Hint: Use dict(zip(keys, values)) to combine lists into dictionary [OK]

Common Mistakes:

• Trying dict(keys, values) directly
• Using zip on dict() results
• Adding lists inside dict()

3. What is the output of this code?

keys = ['a', 'b', 'c']
values = [1, 2, 3]
result = dict(zip(keys, values))
print(result)

medium

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

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

C. [('a', 1), ('b', 2), ('c', 3)]

D. Error: cannot convert zip object to dict

Solution

1. Step 1: Understand zip pairing

   zip(keys, values) pairs 'a' with 1, 'b' with 2, and 'c' with 3.

2. Step 2: Convert pairs to dictionary

   dict() converts these pairs into key-value pairs in a dictionary.

3. Final Answer:

   {'a': 1, 'b': 2, 'c': 3} -> Option A

4. Quick Check:

   dict(zip(keys, values)) = {'a': 1, 'b': 2, 'c': 3} [OK]

Hint: zip pairs keys and values; dict converts pairs to dictionary [OK]

Common Mistakes:

• Confusing keys and values order
• Expecting list instead of dict
• Thinking zip returns dict directly

4. The following code throws an error. What is the problem?

keys = ['x', 'y']
values = [10, 20, 30]
result = dict(zip(keys, values))
print(result)

medium

A. dict() cannot convert zip object

B. zip() cannot be used with lists

C. There is no error; code runs fine

D. The lists have different lengths causing an error

Solution

1. Step 1: Check list lengths and zip behavior

   zip() pairs elements until the shortest list ends, so no error occurs even if lengths differ.

2. Step 2: Confirm dict() accepts zip object

   dict() can convert the zip object to a dictionary without error.

3. Final Answer:

   There is no error; code runs fine -> Option C

4. Quick Check:

   zip truncates to shortest list; dict accepts zip [OK]

Hint: zip stops at shortest list; no error if lengths differ [OK]

Common Mistakes:

• Assuming zip requires equal length lists
• Thinking dict() can't convert zip
• Expecting error due to list length mismatch

5. Given two lists keys = ['name', 'age', 'city'] and values = ['Alice', '', None], which dictionary comprehension correctly creates a dictionary excluding keys with empty or None values?

hard

A. {k: v for k, v in zip(keys, values)}

B. {k: v for k, v in zip(keys, values) if v}

C. {k: v for k, v in zip(keys, values) if v != ''}

D. {k: v for k, v in zip(keys, values) if v is not None}

Solution

1. Step 1: Understand filtering with dictionary comprehension

   We want to exclude keys where values are empty strings or None. The condition if v filters out falsy values like '' and None.

2. Step 2: Analyze each option's filter

   {k: v for k, v in zip(keys, values) if v} uses if v which excludes both '' and None. {k: v for k, v in zip(keys, values) if v is not None} excludes only None, {k: v for k, v in zip(keys, values) if v != ''} excludes only '', and {k: v for k, v in zip(keys, values)} includes all.

3. Final Answer:

   {k: v for k, v in zip(keys, values) if v} -> Option B

4. Quick Check:

   Filter falsy values with if v = D [OK]

Hint: Use if v to exclude empty or None values in comprehension [OK]

Common Mistakes:

• Filtering only None or only empty string
• Not filtering any values
• Using incorrect syntax for filtering