学ぶ Type Conversion Essentials | Cross-Type Interactions

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Type conversion lets you move between core Python types so values can be compared, calculated, or displayed.

Converting to `int`

int(x) makes an integer.

From an int: returns the same number;
From a float: truncates toward zero (for example, int(2.9) returns 2, int(-2.9) returns -2);
From a string: the string must represent an integer (optional spaces and sign are ok).

Valid Conversions


              12345678910
            
# Converting different types of user input to integers
age_input = "  42  "
temperature_reading = 2.9
negative_balance = -2.9

print(int(age_input))            # 42 → clean string converted to int
print(int(temperature_reading))  # 2  → fractional part truncated
print(int(negative_balance))     # -2 → also truncates toward zero
print(int("7"))                  # 7  → string number becomes integer
print(int("   -15  "))           # -15 → handles spaces and sign

These Raise ValueError


              12
            
int("2.5")        # ValueError - not an integer string
int("42a")        # ValueError

Converting to `float`

float(x) makes a floating-point number.

Works for ints and decimal or scientific-notation strings;
Commas are not decimal points in Python.

Valid conversions


              12345678
            
# Converting numeric inputs for a shopping calculator
quantity = 3
price_str = "2.5"
discount_factor = "1e3"  # scientific notation for 1000

print(float(quantity))        # 3.0 → integer converted to float
print(float(price_str))       # 2.5 → string price converted to float
print(float(discount_factor)) # 1000.0 → converts from scientific notation

These Raise ValueError


              1
            
float("2,5")   # ValueError - use a dot, not a comma

Converting to `str`

str(x) makes a human-readable string representation. Prefer f-strings when you are building messages.


              12345678910
            
# Formatting a student's exam result
student_age = 42
average_score = 3.5

print(str(student_age))       # "42" → number converted to string
print(str(average_score))     # "3.5" → float converted to string

student_name, final_score = "Ada", 98
report_message = f"{student_name} scored {final_score} points on the exam."
print(report_message)

Converting to `bool`

bool(x) follows Python truthiness rules.

Numbers: 0 is False, any other number is True;
Strings: "" (empty) is False, any non-empty string is True (even "0" and "False").


              123456789101112
            
# Checking how different user inputs behave as boolean values
login_attempts = 0
notifications = 7
username = ""
user_id = "0"
status = "False"

print(bool(login_attempts))  # False → 0 means no attempts yet
print(bool(notifications))   # True  → non-zero means new notifications
print(bool(username))        # False → empty string means no username entered
print(bool(user_id))         # True  → any non-empty string is truthy
print(bool(status))          # True  → text "False" is still a non-empty string

Mistakes to Avoid

int("2.5") raises ValueError - parse as float() first, then truncate or round;
Locale habit: "2,5" is invalid - use "2.5";
Underscores in input strings: "1_000" is invalid - remove underscores first: "1_000".replace("_", "");
Truthiness surprise: bool("0") is True - compare string contents explicitly if needed, for example s == "0".