Dictionary creation in Python

Introduction

Dictionaries help you store pairs of related information, like names and phone numbers, so you can find things quickly.

When you want to link a word to its meaning, like a mini dictionary.

When you need to store settings with names and values, like volume or brightness.

When you want to count how many times each item appears in a list.

When you want to group data by a key, like students by their grade.

Syntax

Python

my_dict = {key1: value1, key2: value2, key3: value3}

Keys must be unique and usually are strings or numbers.

Values can be any type: numbers, strings, lists, or even other dictionaries.

Examples

This creates an empty dictionary with no items.

Python

empty_dict = {}

A dictionary with keys 'name', 'age', and 'city' storing related information.

Python

person = {"name": "Alice", "age": 30, "city": "New York"}

Keys are subjects and values are scores.

Python

scores = {"math": 90, "science": 85, "english": 88}

Sample Program

This program creates a dictionary to store a student's name, age, and courses. It then prints each piece of information.

Python

student = {
    "name": "John",
    "age": 20,
    "courses": ["Math", "History", "Science"]
}

print(f"Name: {student['name']}")
print(f"Age: {student['age']}")
print(f"Courses: {', '.join(student['courses'])}")

Important Notes

You can create dictionaries using the {} syntax or the dict() function.

Keys must be immutable types like strings, numbers, or tuples.

Dictionaries do not keep order before Python 3.7, but from 3.7+ they remember the order items were added.

Summary

Dictionaries store data as key-value pairs for easy lookup.

Keys are unique and values can be any type.

Use curly braces {} to create dictionaries quickly.

Practice

1. Which of the following is the correct way to create an empty dictionary in Python?

easy

A. my_dict = ()

B. my_dict = []

C. my_dict = {}

D. my_dict = ''

Solution

1. Step 1: Understand dictionary syntax

   Dictionaries in Python are created using curly braces {}.

2. Step 2: Identify the empty dictionary

   An empty dictionary is represented as {}, not square brackets, parentheses, or quotes.

3. Final Answer:

   my_dict = {} -> Option C

4. Quick Check:

   Empty dictionary = {} [OK]

Hint: Use curly braces {} for dictionaries, brackets [] for lists [OK]

Common Mistakes:

• Using [] which creates a list, not a dictionary
• Using () which creates a tuple, not a dictionary
• Using '' which creates an empty string

2. Which of the following is the correct syntax to create a dictionary with keys 'a' and 'b' and values 1 and 2 respectively?

easy

A. my_dict = ['a':1, 'b':2]

B. my_dict = {'a'=1, 'b'=2}

C. my_dict = ('a':1, 'b':2)

D. my_dict = {'a':1, 'b':2}

Solution

1. Step 1: Check dictionary key-value pair syntax

   Dictionary pairs use colon : between key and value inside curly braces.

2. Step 2: Identify correct syntax

   my_dict = {'a':1, 'b':2} uses curly braces and colons correctly. Options A and C use wrong brackets, and D uses equals sign which is invalid.

3. Final Answer:

   my_dict = {'a':1, 'b':2} -> Option D

4. Quick Check:

   Dictionary pairs use : inside {} [OK]

Hint: Use colons : between keys and values inside {} [OK]

Common Mistakes:

• Using square brackets [] instead of curly braces {}
• Using parentheses () instead of curly braces {}
• Using equals sign = instead of colon :

3. What will be the output of the following code?

my_dict = {1: 'one', 2: 'two', 3: 'three'}
print(my_dict[2])

medium

A. 'two'

B. KeyError

C. 2

D. 'one'

Solution

1. Step 1: Understand dictionary key lookup

   Accessing my_dict[2] retrieves the value for key 2.

2. Step 2: Find value for key 2

   Key 2 maps to the string 'two' in the dictionary.

3. Final Answer:

   'two' -> Option A

4. Quick Check:

   my_dict[2] = 'two' [OK]

Hint: Dictionary[key] returns the value for that key [OK]

Common Mistakes:

• Confusing keys and values
• Expecting the key itself as output
• Mistaking KeyError when key exists

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

my_dict = {1: 'one', 2: 'two', 3: 'three'}
print(my_dict[4])

medium

A. Key 4 does not exist in the dictionary

B. Syntax error in dictionary creation

C. Values must be integers, not strings

D. Dictionary keys must be strings

Solution

1. Step 1: Check dictionary keys

   The dictionary has keys 1, 2, and 3 only.

2. Step 2: Accessing a missing key

   Trying to access my_dict[4] causes a KeyError because key 4 is not present.

3. Final Answer:

   Key 4 does not exist in the dictionary -> Option A

4. Quick Check:

   Accessing missing key causes KeyError [OK]

Hint: Check if key exists before accessing dictionary [OK]

Common Mistakes:

• Assuming all keys exist
• Confusing syntax error with runtime error
• Thinking keys must be strings

5. You want to create a dictionary from two lists: keys = ['name', 'age', 'city'] and values = ['Alice', 30, 'NY']. Which code correctly creates this dictionary?

hard

A. my_dict = {keys: values}

B. my_dict = {keys[i]: values[i] for i in range(len(keys))}

C. my_dict = dict(keys, values)

D. my_dict = dict(zip(values, keys))

Solution

1. Step 1: Understand dictionary creation from two lists

   We need to pair each key with its corresponding value by index.

2. Step 2: Analyze options

   my_dict = {keys[i]: values[i] for i in range(len(keys))} uses dictionary comprehension with index to pair keys and values correctly. my_dict = dict(keys, values) is invalid syntax. my_dict = {keys: values} creates a dictionary with one key (the list) which is invalid. my_dict = dict(zip(values, keys)) reverses keys and values.

3. Final Answer:

   my_dict = {keys[i]: values[i] for i in range(len(keys))} -> Option B

4. Quick Check:

   Use dict comprehension with index to pair keys and values [OK]

Hint: Use dict comprehension with index or zip() to pair keys and values [OK]

Common Mistakes:

• Using dict() with two lists directly
• Swapping keys and values in zip()
• Using list as a key