Type conversion (int, float, string) in Python

Introduction

Type conversion helps you change data from one kind to another, like turning numbers into words or words into numbers. This is useful because computers treat different types of data differently.

When you get input from a user as text but need to use it as a number for math.

When you want to show a number as part of a sentence or message.

When you need to store or send data in a specific format, like saving a number as text.

When combining different types in calculations or printing, and they must match.

When reading data from files or the internet that comes as text but should be numbers.

Syntax

Python

int(value)
float(value)
str(value)

int() converts to whole numbers (no decimals).

float() converts to numbers with decimals.

str() converts any value to text.

Examples

This changes text that looks like a number into an actual number you can use in math.

Python

int('123')  # converts string '123' to number 123

This changes text with decimals into a decimal number.

Python

float('3.14')  # converts string '3.14' to number 3.14

This changes a number into text so you can join it with other text.

Python

str(100)  # converts number 100 to string '100'

When converting from float to int, the decimal part is removed, not rounded.

Python

int(3.99)  # converts float 3.99 to int 3 (drops decimals)

Sample Program

This program shows how to convert text input to numbers and numbers to text for printing.

Python

user_input = '42'
number = int(user_input)
print(f"User input as number: {number}")

pi_text = '3.1415'
pi_number = float(pi_text)
print(f"Pi as number: {pi_number}")

score = 99
score_text = str(score)
print("Score as text: " + score_text)

Important Notes

Trying to convert text that is not a number (like 'hello') to int or float will cause an error.

Converting float to int removes decimals without rounding.

Use type conversion to avoid errors when mixing types in operations or printing.

Summary

Type conversion changes data between numbers and text.

Use int(), float(), and str() to convert types.

Be careful with invalid conversions to avoid errors.

Practice

1. What does the Python function int() do when applied to a string like '123'?

easy

A. Converts the string to the integer 123

B. Converts the string to the float 123.0

C. Leaves the string unchanged

D. Causes a syntax error

Solution

1. Step 1: Understand int() function behavior

   The int() function converts a string containing digits into an integer number.

2. Step 2: Apply int() to '123'

   Applying int('123') converts the string to the integer 123.

3. Final Answer:

   Converts the string to the integer 123 -> Option A

4. Quick Check:

   int('123') = 123 [OK]

Hint: int() turns digit strings into whole numbers [OK]

Common Mistakes:

• Thinking int() converts to float
• Expecting string unchanged
• Confusing with syntax error

2. Which of the following is the correct syntax to convert the float number 3.14 to an integer in Python?

easy

A. int(3,14)

B. int '3.14'

C. int(3.14)

D. int(3.14.0)

Solution

1. Step 1: Recall correct function call syntax

   Functions in Python require parentheses around arguments, e.g., int(3.14).

2. Step 2: Check each option

   int(3.14) uses correct syntax. Others have syntax errors or invalid argument formats.

3. Final Answer:

   int(3.14) -> Option C

4. Quick Check:

   int(3.14) is valid syntax [OK]

Hint: Use parentheses with int() for conversion [OK]

Common Mistakes:

• Omitting parentheses
• Using commas instead of dots
• Adding extra dots in numbers

3. What is the output of this code?

value = '45.67'
result = float(value)
print(int(result))

medium

A. 46

B. 45

C. Error

D. 45.67

Solution

1. Step 1: Convert string to float

   float('45.67') converts the string to the float number 45.67.

2. Step 2: Convert float to int

   int(45.67) converts the float to integer by dropping the decimal part, resulting in 45.

3. Final Answer:

   45 -> Option B

4. Quick Check:

   int(float('45.67')) = 45 [OK]

Hint: int() drops decimals, does not round [OK]

Common Mistakes:

• Expecting rounding to 46
• Printing float instead of int
• Confusing string and number types

4. The code below causes an error. What is the fix?

num = 'abc'
converted = int(num)
print(converted)

medium

A. Use float() instead of int()

B. Add quotes around int

C. Remove the int() conversion

D. Change 'abc' to a string of digits like '123'

Solution

1. Step 1: Identify cause of error

   Trying to convert 'abc' to int causes a ValueError because 'abc' is not numeric.

2. Step 2: Fix by using numeric string

   Replacing 'abc' with a string of digits like '123' allows int() to convert successfully.

3. Final Answer:

   Change 'abc' to a string of digits like '123' -> Option D

4. Quick Check:

   int('123') works, int('abc') errors [OK]

Hint: int() needs numeric strings, not letters [OK]

Common Mistakes:

• Using float() on non-numeric string
• Removing conversion but expecting int
• Adding quotes around function name

5. You have a list of mixed strings representing numbers: ['10', '20.5', '30', '40.0']. How do you convert all to integers correctly in Python?

hard

A. Use [int(float(x)) for x in list]

B. Use [int(x) for x in list]

C. Use [float(int(x)) for x in list]

D. Use [str(int(x)) for x in list]

Solution

1. Step 1: Understand list contents

   The list has strings representing integers and floats, e.g., '20.5' and '40.0'.

2. Step 2: Convert strings to float first

   Direct int() on '20.5' causes error, so convert to float first: float('20.5') = 20.5.

3. Step 3: Convert float to int

   Then convert float to int with int(20.5) = 20, dropping decimals.

4. Final Answer:

   Use [int(float(x)) for x in list] -> Option A

5. Quick Check:

   int(float('20.5')) = 20 [OK]

Hint: Convert string to float, then to int for decimals [OK]

Common Mistakes:

• Trying int() directly on float strings
• Swapping int and float conversions
• Converting to string instead of int