Data Types

Type Conversion

Changing Data Types

A web form returns the user's age as text: "25". To check if they're old enough to vote (age >= 18), you need to convert "25" to the number 25. Type conversion bridges the gap between data formats.

Int to float (precise division)

When dividing integers, you might want a decimal result.

example
int_to_float.py
Replay: real traced execution (multi-file project)
a = 7
b = 3

# Python 3: / always returns float
result = a / b
print(f"Division: {a}/{b} = {result}")

# Integer division with //
int_result = a // b
print(f"Integer division: {a}//{b} = {int_result}")

# Explicit conversion (not needed in Python 3 for division)
float_result = float(a) / float(b)
print(f"Explicit float: {float_result}")

# Practical: calculate percentage
score = 85
total = 100
percentage = score / total * 100
print(f"Percentage: {percentage}%")

a = 5
b = 3

# Python 3: / always returns float
result = a / b
print(f"Division: {a}/{b} = {result}")

# Integer division with //
int_result = a // b
print(f"Integer division: {a}//{b} = {int_result}")

# Explicit conversion (not needed in Python 3 for division)
float_result = float(a) / float(b)
print(f"Explicit float: {float_result}")

# Practical: calculate percentage
score = 85
total = 100
percentage = score / total * 100
print(f"Percentage: {percentage}%")

a = 10
b = 3

# Python 3: / always returns float
result = a / b
print(f"Division: {a}/{b} = {result}")

# Integer division with //
int_result = a // b
print(f"Integer division: {a}//{b} = {int_result}")

# Explicit conversion (not needed in Python 3 for division)
float_result = float(a) / float(b)
print(f"Explicit float: {float_result}")

# Practical: calculate percentage
score = 85
total = 100
percentage = score / total * 100
print(f"Percentage: {percentage}%")

a = 7
b = 2

# Python 3: / always returns float
result = a / b
print(f"Division: {a}/{b} = {result}")

# Integer division with //
int_result = a // b
print(f"Integer division: {a}//{b} = {int_result}")

# Explicit conversion (not needed in Python 3 for division)
float_result = float(a) / float(b)
print(f"Explicit float: {float_result}")

# Practical: calculate percentage
score = 85
total = 100
percentage = score / total * 100
print(f"Percentage: {percentage}%")

a = 7
b = 4

# Python 3: / always returns float
result = a / b
print(f"Division: {a}/{b} = {result}")

# Integer division with //
int_result = a // b
print(f"Integer division: {a}//{b} = {int_result}")

# Explicit conversion (not needed in Python 3 for division)
float_result = float(a) / float(b)
print(f"Explicit float: {float_result}")

# Practical: calculate percentage
score = 85
total = 100
percentage = score / total * 100
print(f"Percentage: {percentage}%")

  1. a ← 7, b ← 3, result ← 2.3333333333333335, int_result ← 2, float_result ← 2.3333333333333335

    1a→ 7 = 7   #@a=5, 102b→ 3 = 3   #@b=2, 434# Python 3: / always returns float5result→ 2.3333333333333335 = a7 / b36print(f"Division: {a7}/{b3} = {result2.3333333333333335}")78# Integer division with //9int_result→ 2 = a7 // b310print(f"Integer division: {a7}//{b3} = {int_result2}")1112# Explicit conversion (not needed in Python 3 for division)13float_result→ 2.3333333333333335 = float(a7) / float(b3)14print(f"Explicit float: {float_result2.3333333333333335}")1516# Practical: calculate percentage17score→ 85 = 8518total→ 100 = 10019percentage→ 85.0 = score85 / total100 * 10020print(f"Percentage: {percentage85.0}%")
    outputDivision: 7/3 = 2.3333333333333335
Integer division: 7//3 = 2
Explicit float: 2.3333333333333335
Percentage: 85.0%

  1. a ← 5, b ← 3, result ← 1.6666666666666667, int_result ← 1, float_result ← 1.6666666666666667

    1a→ 5 = 52b→ 3 = 334# Python 3: / always returns float5result→ 1.6666666666666667 = a5 / b36print(f"Division: {a5}/{b3} = {result1.6666666666666667}")78# Integer division with //9int_result→ 1 = a5 // b310print(f"Integer division: {a5}//{b3} = {int_result1}")1112# Explicit conversion (not needed in Python 3 for division)13float_result→ 1.6666666666666667 = float(a5) / float(b3)14print(f"Explicit float: {float_result1.6666666666666667}")1516# Practical: calculate percentage17score→ 85 = 8518total→ 100 = 10019percentage→ 85.0 = score85 / total100 * 10020print(f"Percentage: {percentage85.0}%")
    outputDivision: 5/3 = 1.6666666666666667
Integer division: 5//3 = 1
Explicit float: 1.6666666666666667
Percentage: 85.0%

  1. a ← 10, b ← 3, result ← 3.3333333333333335, int_result ← 3, float_result ← 3.3333333333333335

    1a→ 10 = 102b→ 3 = 334# Python 3: / always returns float5result→ 3.3333333333333335 = a10 / b36print(f"Division: {a10}/{b3} = {result3.3333333333333335}")78# Integer division with //9int_result→ 3 = a10 // b310print(f"Integer division: {a10}//{b3} = {int_result3}")1112# Explicit conversion (not needed in Python 3 for division)13float_result→ 3.3333333333333335 = float(a10) / float(b3)14print(f"Explicit float: {float_result3.3333333333333335}")1516# Practical: calculate percentage17score→ 85 = 8518total→ 100 = 10019percentage→ 85.0 = score85 / total100 * 10020print(f"Percentage: {percentage85.0}%")
    outputDivision: 10/3 = 3.3333333333333335
Integer division: 10//3 = 3
Explicit float: 3.3333333333333335
Percentage: 85.0%

  1. a ← 7, b ← 2, result ← 3.5, int_result ← 3, float_result ← 3.5

    1a→ 7 = 72b→ 2 = 234# Python 3: / always returns float5result→ 3.5 = a7 / b26print(f"Division: {a7}/{b2} = {result3.5}")78# Integer division with //9int_result→ 3 = a7 // b210print(f"Integer division: {a7}//{b2} = {int_result3}")1112# Explicit conversion (not needed in Python 3 for division)13float_result→ 3.5 = float(a7) / float(b2)14print(f"Explicit float: {float_result3.5}")1516# Practical: calculate percentage17score→ 85 = 8518total→ 100 = 10019percentage→ 85.0 = score85 / total100 * 10020print(f"Percentage: {percentage85.0}%")
    outputDivision: 7/2 = 3.5
Integer division: 7//2 = 3
Explicit float: 3.5
Percentage: 85.0%

  1. a ← 7, b ← 4, result ← 1.75, int_result ← 1, float_result ← 1.75

    1a→ 7 = 72b→ 4 = 434# Python 3: / always returns float5result→ 1.75 = a7 / b46print(f"Division: {a7}/{b4} = {result1.75}")78# Integer division with //9int_result→ 1 = a7 // b410print(f"Integer division: {a7}//{b4} = {int_result1}")1112# Explicit conversion (not needed in Python 3 for division)13float_result→ 1.75 = float(a7) / float(b4)14print(f"Explicit float: {float_result1.75}")1516# Practical: calculate percentage17score→ 85 = 8518total→ 100 = 10019percentage→ 85.0 = score85 / total100 * 10020print(f"Percentage: {percentage85.0}%")
    outputDivision: 7/4 = 1.75
Integer division: 7//4 = 1
Explicit float: 1.75
Percentage: 85.0%

Python 3's / operator always returns float. Use // for integer division.

Float to int (truncation)

Sometimes you need a whole number from a decimal.

price
float_to_int.py
Replay: real traced execution (multi-file project)
price = 19.99

# int() truncates (drops decimals)
truncated = int(price)
print(f"Price: {price}")
print(f"Truncated: {truncated}")

# round() rounds to nearest
rounded = round(price)
print(f"Rounded: {rounded}")

# round() can specify decimal places
two_decimals = round(price, 1)
print(f"One decimal: {two_decimals}")

# Floor and ceiling
import math
floor_val = math.floor(price)
ceiling_val = math.ceil(price)
print(f"Floor: {floor_val}")
print(f"Ceiling: {ceiling_val}")

# Practical: convert dollars to cents (avoid floating point)
cents = round(price * 100)
print(f"Cents: {cents}")

price = 25.50

# int() truncates (drops decimals)
truncated = int(price)
print(f"Price: {price}")
print(f"Truncated: {truncated}")

# round() rounds to nearest
rounded = round(price)
print(f"Rounded: {rounded}")

# round() can specify decimal places
two_decimals = round(price, 1)
print(f"One decimal: {two_decimals}")

# Floor and ceiling
import math
floor_val = math.floor(price)
ceiling_val = math.ceil(price)
print(f"Floor: {floor_val}")
print(f"Ceiling: {ceiling_val}")

# Practical: convert dollars to cents (avoid floating point)
cents = round(price * 100)
print(f"Cents: {cents}")

price = 9.95

# int() truncates (drops decimals)
truncated = int(price)
print(f"Price: {price}")
print(f"Truncated: {truncated}")

# round() rounds to nearest
rounded = round(price)
print(f"Rounded: {rounded}")

# round() can specify decimal places
two_decimals = round(price, 1)
print(f"One decimal: {two_decimals}")

# Floor and ceiling
import math
floor_val = math.floor(price)
ceiling_val = math.ceil(price)
print(f"Floor: {floor_val}")
print(f"Ceiling: {ceiling_val}")

# Practical: convert dollars to cents (avoid floating point)
cents = round(price * 100)
print(f"Cents: {cents}")

  1. price ← 19.99, truncated ← 19, rounded ← 20, two_decimals ← 20.0

    1price→ 19.99 = 19.99   #@price=25.50, 9.9523# int() truncates (drops decimals)4truncated→ 19 = int(price19.99)5print(f"Price: {price19.99}")6print(f"Truncated: {truncated19}")78# round() rounds to nearest9rounded→ 20 = round(price19.99)10print(f"Rounded: {rounded20}")1112# round() can specify decimal places13two_decimals→ 20.0 = round(price19.99, 1)14print(f"One decimal: {two_decimals20.0}")1516# Floor and ceiling17import math18floor_val→ 19 = math<module 'math' from '/usr/local/lib/python3.12/lib-dynload/math.cpython-312-x86_64-linux-gnu.so'>.floor(price19.99)19ceiling_val→ 20 = math<module 'math' from '/usr/local/lib/python3.12/lib-dynload/math.cpython-312-x86_64-linux-gnu.so'>.ceil(price19.99)20print(f"Floor: {floor_val19}")21print(f"Ceiling: {ceiling_val20}")2223# Practical: convert dollars to cents (avoid floating point)24cents→ 1999 = round(price19.99 * 100)25print(f"Cents: {cents1999}")
    outputPrice: 19.99
Truncated: 19
Rounded: 20
One decimal: 20.0
Floor: 19
Ceiling: 20
Cents: 1999

  1. price ← 25.5, truncated ← 25, rounded ← 26, two_decimals ← 25.5

    1price→ 25.5 = 25.5023# int() truncates (drops decimals)4truncated→ 25 = int(price25.5)5print(f"Price: {price25.5}")6print(f"Truncated: {truncated25}")78# round() rounds to nearest9rounded→ 26 = round(price25.5)10print(f"Rounded: {rounded26}")1112# round() can specify decimal places13two_decimals→ 25.5 = round(price25.5, 1)14print(f"One decimal: {two_decimals25.5}")1516# Floor and ceiling17import math18floor_val→ 25 = math<module 'math' from '/usr/local/lib/python3.12/lib-dynload/math.cpython-312-x86_64-linux-gnu.so'>.floor(price25.5)19ceiling_val→ 26 = math<module 'math' from '/usr/local/lib/python3.12/lib-dynload/math.cpython-312-x86_64-linux-gnu.so'>.ceil(price25.5)20print(f"Floor: {floor_val25}")21print(f"Ceiling: {ceiling_val26}")2223# Practical: convert dollars to cents (avoid floating point)24cents→ 2550 = round(price25.5 * 100)25print(f"Cents: {cents2550}")
    outputPrice: 25.5
Truncated: 25
Rounded: 26
One decimal: 25.5
Floor: 25
Ceiling: 26
Cents: 2550

  1. price ← 9.95, truncated ← 9, rounded ← 10, two_decimals ← 9.9

    1price→ 9.95 = 9.9523# int() truncates (drops decimals)4truncated→ 9 = int(price9.95)5print(f"Price: {price9.95}")6print(f"Truncated: {truncated9}")78# round() rounds to nearest9rounded→ 10 = round(price9.95)10print(f"Rounded: {rounded10}")1112# round() can specify decimal places13two_decimals→ 9.9 = round(price9.95, 1)14print(f"One decimal: {two_decimals9.9}")1516# Floor and ceiling17import math18floor_val→ 9 = math<module 'math' from '/usr/local/lib/python3.12/lib-dynload/math.cpython-312-x86_64-linux-gnu.so'>.floor(price9.95)19ceiling_val→ 10 = math<module 'math' from '/usr/local/lib/python3.12/lib-dynload/math.cpython-312-x86_64-linux-gnu.so'>.ceil(price9.95)20print(f"Floor: {floor_val9}")21print(f"Ceiling: {ceiling_val10}")2223# Practical: convert dollars to cents (avoid floating point)24cents→ 995 = round(price9.95 * 100)25print(f"Cents: {cents995}")
    outputPrice: 9.95
Truncated: 9
Rounded: 10
One decimal: 9.9
Floor: 9
Ceiling: 10
Cents: 995
int() Convert to integer: `int("42")` → `42`, `int(3.7)` → `3` (truncates, doesn't round)

String to number (parse input)

User input is text. Convert to numbers for calculation.

example
parse.py
Replay: real traced execution (multi-file project)
# Simulated user input
age_input = "25"
price_input = "19.99"

# Parse string to number
age = int(age_input)
price = float(price_input)

print(f"Age string: {age_input}")
print(f"Age number: {age}")
print(f"Age + 1 = {age + 1}")

print(f"Price string: {price_input}")
print(f"Price number: {price}")
print(f"With tax: {price * 1.1}")

# Complex number from string
complex_str = "3+4j"
c = complex(complex_str)
print(f"Complex: {c}, magnitude: {abs(c)}")

# Simulated user input
age_input = "30"
price_input = "19.99"

# Parse string to number
age = int(age_input)
price = float(price_input)

print(f"Age string: {age_input}")
print(f"Age number: {age}")
print(f"Age + 1 = {age + 1}")

print(f"Price string: {price_input}")
print(f"Price number: {price}")
print(f"With tax: {price * 1.1}")

# Complex number from string
complex_str = "3+4j"
c = complex(complex_str)
print(f"Complex: {c}, magnitude: {abs(c)}")

# Simulated user input
age_input = "17"
price_input = "19.99"

# Parse string to number
age = int(age_input)
price = float(price_input)

print(f"Age string: {age_input}")
print(f"Age number: {age}")
print(f"Age + 1 = {age + 1}")

print(f"Price string: {price_input}")
print(f"Price number: {price}")
print(f"With tax: {price * 1.1}")

# Complex number from string
complex_str = "3+4j"
c = complex(complex_str)
print(f"Complex: {c}, magnitude: {abs(c)}")

# Simulated user input
age_input = "25"
price_input = "9.95"

# Parse string to number
age = int(age_input)
price = float(price_input)

print(f"Age string: {age_input}")
print(f"Age number: {age}")
print(f"Age + 1 = {age + 1}")

print(f"Price string: {price_input}")
print(f"Price number: {price}")
print(f"With tax: {price * 1.1}")

# Complex number from string
complex_str = "3+4j"
c = complex(complex_str)
print(f"Complex: {c}, magnitude: {abs(c)}")

# Simulated user input
age_input = "25"
price_input = "100.00"

# Parse string to number
age = int(age_input)
price = float(price_input)

print(f"Age string: {age_input}")
print(f"Age number: {age}")
print(f"Age + 1 = {age + 1}")

print(f"Price string: {price_input}")
print(f"Price number: {price}")
print(f"With tax: {price * 1.1}")

# Complex number from string
complex_str = "3+4j"
c = complex(complex_str)
print(f"Complex: {c}, magnitude: {abs(c)}")

  1. age_input ← 25, price_input ← 19.99, age ← 25, price ← 19.99, complex_str ← 3+4j

    1# Simulated user input2age_input→ 25 = "25"       #@age_input="30", "17"3price_input→ 19.99 = "19.99"  #@price_input="9.95", "100.00"45# Parse string to number6age→ 25 = int(age_input25)7price→ 19.99 = float(price_input19.99)89print(f"Age string: {age_input25}")10print(f"Age number: {age25}")11print(f"Age + 1 = {age25 + 1}")1213print(f"Price string: {price_input19.99}")14print(f"Price number: {price19.99}")15print(f"With tax: {price19.99 * 1.1}")1617# Complex number from string18complex_str→ 3+4j = "3+4j"19c→ (3+4j) = complex(complex_str3+4j)20print(f"Complex: {c(3+4j)}, magnitude: {abs(c)}")
    outputAge string: 25
Age number: 25
Age + 1 = 26
Price string: 19.99
Price number: 19.99
With tax: 21.989
Complex: (3+4j), magnitude: 5.0

  1. age_input ← 30, price_input ← 19.99, age ← 30, price ← 19.99, complex_str ← 3+4j

    1# Simulated user input2age_input→ 30 = "30"3price_input→ 19.99 = "19.99"45# Parse string to number6age→ 30 = int(age_input30)7price→ 19.99 = float(price_input19.99)89print(f"Age string: {age_input30}")10print(f"Age number: {age30}")11print(f"Age + 1 = {age30 + 1}")1213print(f"Price string: {price_input19.99}")14print(f"Price number: {price19.99}")15print(f"With tax: {price19.99 * 1.1}")1617# Complex number from string18complex_str→ 3+4j = "3+4j"19c→ (3+4j) = complex(complex_str3+4j)20print(f"Complex: {c(3+4j)}, magnitude: {abs(c)}")
    outputAge string: 30
Age number: 30
Age + 1 = 31
Price string: 19.99
Price number: 19.99
With tax: 21.989
Complex: (3+4j), magnitude: 5.0

  1. age_input ← 17, price_input ← 19.99, age ← 17, price ← 19.99, complex_str ← 3+4j

    1# Simulated user input2age_input→ 17 = "17"3price_input→ 19.99 = "19.99"45# Parse string to number6age→ 17 = int(age_input17)7price→ 19.99 = float(price_input19.99)89print(f"Age string: {age_input17}")10print(f"Age number: {age17}")11print(f"Age + 1 = {age17 + 1}")1213print(f"Price string: {price_input19.99}")14print(f"Price number: {price19.99}")15print(f"With tax: {price19.99 * 1.1}")1617# Complex number from string18complex_str→ 3+4j = "3+4j"19c→ (3+4j) = complex(complex_str3+4j)20print(f"Complex: {c(3+4j)}, magnitude: {abs(c)}")
    outputAge string: 17
Age number: 17
Age + 1 = 18
Price string: 19.99
Price number: 19.99
With tax: 21.989
Complex: (3+4j), magnitude: 5.0

  1. age_input ← 25, price_input ← 9.95, age ← 25, price ← 9.95, complex_str ← 3+4j

    1# Simulated user input2age_input→ 25 = "25"3price_input→ 9.95 = "9.95"45# Parse string to number6age→ 25 = int(age_input25)7price→ 9.95 = float(price_input9.95)89print(f"Age string: {age_input25}")10print(f"Age number: {age25}")11print(f"Age + 1 = {age25 + 1}")1213print(f"Price string: {price_input9.95}")14print(f"Price number: {price9.95}")15print(f"With tax: {price9.95 * 1.1}")1617# Complex number from string18complex_str→ 3+4j = "3+4j"19c→ (3+4j) = complex(complex_str3+4j)20print(f"Complex: {c(3+4j)}, magnitude: {abs(c)}")
    outputAge string: 25
Age number: 25
Age + 1 = 26
Price string: 9.95
Price number: 9.95
With tax: 10.945
Complex: (3+4j), magnitude: 5.0

  1. age_input ← 25, price_input ← 100.00, age ← 25, price ← 100.0

    1# Simulated user input2age_input→ 25 = "25"3price_input→ 100.00 = "100.00"45# Parse string to number6age→ 25 = int(age_input25)7price→ 100.0 = float(price_input100.00)89print(f"Age string: {age_input25}")10print(f"Age number: {age25}")11print(f"Age + 1 = {age25 + 1}")1213print(f"Price string: {price_input100.00}")14print(f"Price number: {price100.0}")15print(f"With tax: {price100.0 * 1.1}")1617# Complex number from string18complex_str→ 3+4j = "3+4j"19c→ (3+4j) = complex(complex_str3+4j)20print(f"Complex: {c(3+4j)}, magnitude: {abs(c)}")
    outputAge string: 25
Age number: 25
Age + 1 = 26
Price string: 100.00
Price number: 100.0
With tax: 110.00000000000001
Complex: (3+4j), magnitude: 5.0

See the Type Change

The conversion call changes what operations are possible. These diagrams pin the exact string inputs used in parse.py.

Text input becomes numbersText input becomes numbers"25"int()25"19.99"float()19.99
`age_input` starts as the text "25" and becomes integer 25. `price_input` starts as the text "19.99" and becomes float 19.99.
Converted values unlock calculationsConverted values unlock calculationsage=25age+126price=19.99*1.121.989
After conversion, Python can add 1 to age and multiply price by 1.1. Before conversion, the inputs are just text.
float() Convert to float: `float("3.14")` → `3.14`, `float(5)` → `5.0`

Number to string (format output)

Convert numbers to strings for display or formatting.

count
to_string.py
Replay: real traced execution (multi-file project)
count = 42
price = 19.99
active = True

# Explicit conversion with str()
count_str = str(count)
price_str = str(price)
active_str = str(active)

print(f"Count as string: '{count_str}'")
print(f"Price as string: '{price_str}'")
print(f"Active as string: '{active_str}'")

# F-string formatting (preferred)
message = f"You have {count} items"
print(message)

# Format with specific decimal places
formatted = f"Price: ${price:.2f}"
print(formatted)

# repr() for debug representation
text = "Hello\nWorld"
print(f"str():  {str(text)}")
print(f"repr(): {repr(text)}")

count = 7
price = 19.99
active = True

# Explicit conversion with str()
count_str = str(count)
price_str = str(price)
active_str = str(active)

print(f"Count as string: '{count_str}'")
print(f"Price as string: '{price_str}'")
print(f"Active as string: '{active_str}'")

# F-string formatting (preferred)
message = f"You have {count} items"
print(message)

# Format with specific decimal places
formatted = f"Price: ${price:.2f}"
print(formatted)

# repr() for debug representation
text = "Hello\nWorld"
print(f"str():  {str(text)}")
print(f"repr(): {repr(text)}")

count = 100
price = 19.99
active = True

# Explicit conversion with str()
count_str = str(count)
price_str = str(price)
active_str = str(active)

print(f"Count as string: '{count_str}'")
print(f"Price as string: '{price_str}'")
print(f"Active as string: '{active_str}'")

# F-string formatting (preferred)
message = f"You have {count} items"
print(message)

# Format with specific decimal places
formatted = f"Price: ${price:.2f}"
print(formatted)

# repr() for debug representation
text = "Hello\nWorld"
print(f"str():  {str(text)}")
print(f"repr(): {repr(text)}")

  1. count ← 42, price ← 19.99, active ← True, count_str ← 42, price_str ← 19.99

    1count→ 42 = 42  #@count=7, 1002price→ 19.99 = 19.993active→ True = True45# Explicit conversion with str()6count_str→ 42 = str(count42)7price_str→ 19.99 = str(price19.99)8active_str→ True = str(activeTrue)910print(f"Count as string: '{count_str42}'")11print(f"Price as string: '{price_str19.99}'")12print(f"Active as string: '{active_strTrue}'")1314# F-string formatting (preferred)15message→ You have 42 items = f"You have {count42} items"16print(messageYou have 42 items)1718# Format with specific decimal places19formatted→ Price: $19.99 = f"Price: ${price19.99:.2f}"20print(formattedPrice: $19.99)2122# repr() for debug representation23text→ Hello
World = "Hello\nWorld"24print(f"str():  {str(textHello
World)}")25print(f"repr(): {repr(textHello
World)}")
    outputCount as string: '42'
Price as string: '19.99'
Active as string: 'True'
You have 42 items
Price: $19.99
str():  Hello
World
repr(): 'Hello\nWorld'

  1. count ← 7, price ← 19.99, active ← True, count_str ← 7, price_str ← 19.99

    1count→ 7 = 72price→ 19.99 = 19.993active→ True = True45# Explicit conversion with str()6count_str→ 7 = str(count7)7price_str→ 19.99 = str(price19.99)8active_str→ True = str(activeTrue)910print(f"Count as string: '{count_str7}'")11print(f"Price as string: '{price_str19.99}'")12print(f"Active as string: '{active_strTrue}'")1314# F-string formatting (preferred)15message→ You have 7 items = f"You have {count7} items"16print(messageYou have 7 items)1718# Format with specific decimal places19formatted→ Price: $19.99 = f"Price: ${price19.99:.2f}"20print(formattedPrice: $19.99)2122# repr() for debug representation23text→ Hello
World = "Hello\nWorld"24print(f"str():  {str(textHello
World)}")25print(f"repr(): {repr(textHello
World)}")
    outputCount as string: '7'
Price as string: '19.99'
Active as string: 'True'
You have 7 items
Price: $19.99
str():  Hello
World
repr(): 'Hello\nWorld'

  1. count ← 100, price ← 19.99, active ← True, count_str ← 100, price_str ← 19.99

    1count→ 100 = 1002price→ 19.99 = 19.993active→ True = True45# Explicit conversion with str()6count_str→ 100 = str(count100)7price_str→ 19.99 = str(price19.99)8active_str→ True = str(activeTrue)910print(f"Count as string: '{count_str100}'")11print(f"Price as string: '{price_str19.99}'")12print(f"Active as string: '{active_strTrue}'")1314# F-string formatting (preferred)15message→ You have 100 items = f"You have {count100} items"16print(messageYou have 100 items)1718# Format with specific decimal places19formatted→ Price: $19.99 = f"Price: ${price19.99:.2f}"20print(formattedPrice: $19.99)2122# repr() for debug representation23text→ Hello
World = "Hello\nWorld"24print(f"str():  {str(textHello
World)}")25print(f"repr(): {repr(textHello
World)}")
    outputCount as string: '100'
Price as string: '19.99'
Active as string: 'True'
You have 100 items
Price: $19.99
str():  Hello
World
repr(): 'Hello\nWorld'

Use str() or f-strings for conversion with formatting options.

str() Convert to string: `str(42)` → `"42"`

Truthiness conversions

Python converts values to boolean in conditions automatically.

example
truthiness.py
Replay: real traced execution (multi-file project)
# Falsy values in Python
print("=== Falsy Values ===")
print(f"bool(False): {bool(False)}")
print(f"bool(0): {bool(0)}")
print(f"bool(0.0): {bool(0.0)}")
print(f"bool(''): {bool('')}")
print(f"bool([]): {bool([])}")
print(f"bool(None): {bool(None)}")

# Truthy values
print("\n=== Truthy Values ===")
print(f"bool(True): {bool(True)}")
print(f"bool(1): {bool(1)}")
print(f"bool(-1): {bool(-1)}")
print(f"bool('hi'): {bool('hi')}")
print(f"bool([1, 2]): {bool([1, 2])}")

# Practical use in conditions
name = ""
if name:
    print(f"Hello, {name}!")
else:
    print("Name is empty")

items = []
if items:
    print(f"Has {len(items)} items")
else:
    print("List is empty")


# Falsy values in Python
print("=== Falsy Values ===")
print(f"bool(False): {bool(False)}")
print(f"bool(0): {bool(0)}")
print(f"bool(0.0): {bool(0.0)}")
print(f"bool(''): {bool('')}")
print(f"bool([]): {bool([])}")
print(f"bool(None): {bool(None)}")

# Truthy values
print("\n=== Truthy Values ===")
print(f"bool(True): {bool(True)}")
print(f"bool(1): {bool(1)}")
print(f"bool(-1): {bool(-1)}")
print(f"bool('hi'): {bool('hi')}")
print(f"bool([1, 2]): {bool([1, 2])}")

# Practical use in conditions
name = "Alice"
if name:
    print(f"Hello, {name}!")
else:
    print("Name is empty")

items = []
if items:
    print(f"Has {len(items)} items")
else:
    print("List is empty")


# Falsy values in Python
print("=== Falsy Values ===")
print(f"bool(False): {bool(False)}")
print(f"bool(0): {bool(0)}")
print(f"bool(0.0): {bool(0.0)}")
print(f"bool(''): {bool('')}")
print(f"bool([]): {bool([])}")
print(f"bool(None): {bool(None)}")

# Truthy values
print("\n=== Truthy Values ===")
print(f"bool(True): {bool(True)}")
print(f"bool(1): {bool(1)}")
print(f"bool(-1): {bool(-1)}")
print(f"bool('hi'): {bool('hi')}")
print(f"bool([1, 2]): {bool([1, 2])}")

# Practical use in conditions
name = "Bob"
if name:
    print(f"Hello, {name}!")
else:
    print("Name is empty")

items = []
if items:
    print(f"Has {len(items)} items")
else:
    print("List is empty")


# Falsy values in Python
print("=== Falsy Values ===")
print(f"bool(False): {bool(False)}")
print(f"bool(0): {bool(0)}")
print(f"bool(0.0): {bool(0.0)}")
print(f"bool(''): {bool('')}")
print(f"bool([]): {bool([])}")
print(f"bool(None): {bool(None)}")

# Truthy values
print("\n=== Truthy Values ===")
print(f"bool(True): {bool(True)}")
print(f"bool(1): {bool(1)}")
print(f"bool(-1): {bool(-1)}")
print(f"bool('hi'): {bool('hi')}")
print(f"bool([1, 2]): {bool([1, 2])}")

# Practical use in conditions
name = ""
if name:
    print(f"Hello, {name}!")
else:
    print("Name is empty")

items = [1, 2, 3]
if items:
    print(f"Has {len(items)} items")
else:
    print("List is empty")

  1. name ← (empty)

    1# Falsy values in Python2print("=== Falsy Values ===")3print(f"bool(False): {bool(False)}")4print(f"bool(0): {bool(0)}")5print(f"bool(0.0): {bool(0.0)}")6print(f"bool(''): {bool('')}")7print(f"bool([]): {bool([])}")8print(f"bool(None): {bool(None)}")910# Truthy values11print("\n=== Truthy Values ===")12print(f"bool(True): {bool(True)}")13print(f"bool(1): {bool(1)}")14print(f"bool(-1): {bool(-1)}")15print(f"bool('hi'): {bool('hi')}")16print(f"bool([1, 2]): {bool([1, 2])}")1718# Practical use in conditions19name→ (empty) = ""  #@name="Alice", "Bob"20if name:  #?truthiness_check
    output=== Falsy Values ===
bool(False): False
bool(0): False
bool(0.0): False
bool(''): False
bool([]): False
bool(None): False

=== Truthy Values ===
bool(True): True
bool(1): True
bool(-1): True
bool('hi'): True
bool([1, 2]): True

  2. else:

    20if name:  #?truthiness_check21    print(f"Hello, {name}!")22else:23    print("Name is empty")
    outputName is empty

  3. items ← []

    25items→ [] = []  #@items=[1, 2, 3]26if items:

  4. else:

    26if items:27    print(f"Has {len(items)} items")28else:29    print("List is empty")
    outputList is empty

  1. name ← Alice

    1# Falsy values in Python2print("=== Falsy Values ===")3print(f"bool(False): {bool(False)}")4print(f"bool(0): {bool(0)}")5print(f"bool(0.0): {bool(0.0)}")6print(f"bool(''): {bool('')}")7print(f"bool([]): {bool([])}")8print(f"bool(None): {bool(None)}")910# Truthy values11print("\n=== Truthy Values ===")12print(f"bool(True): {bool(True)}")13print(f"bool(1): {bool(1)}")14print(f"bool(-1): {bool(-1)}")15print(f"bool('hi'): {bool('hi')}")16print(f"bool([1, 2]): {bool([1, 2])}")1718# Practical use in conditions19name→ Alice = "Alice"20if name:
    output=== Falsy Values ===
bool(False): False
bool(0): False
bool(0.0): False
bool(''): False
bool([]): False
bool(None): False

=== Truthy Values ===
bool(True): True
bool(1): True
bool(-1): True
bool('hi'): True
bool([1, 2]): True

  2. if name:

    19name = "Alice"20if nameAlice:21    print(f"Hello, {nameAlice}!")22else:
    outputHello, Alice!

  3. items ← []

    25items→ [] = []26if items:

  4. else:

    26if items:27    print(f"Has {len(items)} items")28else:29    print("List is empty")
    outputList is empty

  1. name ← Bob

    1# Falsy values in Python2print("=== Falsy Values ===")3print(f"bool(False): {bool(False)}")4print(f"bool(0): {bool(0)}")5print(f"bool(0.0): {bool(0.0)}")6print(f"bool(''): {bool('')}")7print(f"bool([]): {bool([])}")8print(f"bool(None): {bool(None)}")910# Truthy values11print("\n=== Truthy Values ===")12print(f"bool(True): {bool(True)}")13print(f"bool(1): {bool(1)}")14print(f"bool(-1): {bool(-1)}")15print(f"bool('hi'): {bool('hi')}")16print(f"bool([1, 2]): {bool([1, 2])}")1718# Practical use in conditions19name→ Bob = "Bob"20if name:
    output=== Falsy Values ===
bool(False): False
bool(0): False
bool(0.0): False
bool(''): False
bool([]): False
bool(None): False

=== Truthy Values ===
bool(True): True
bool(1): True
bool(-1): True
bool('hi'): True
bool([1, 2]): True

  2. if name:

    19name = "Bob"20if nameBob:21    print(f"Hello, {nameBob}!")22else:
    outputHello, Bob!

  3. items ← []

    25items→ [] = []26if items:

  4. else:

    26if items:27    print(f"Has {len(items)} items")28else:29    print("List is empty")
    outputList is empty

  1. name ← (empty)

    1# Falsy values in Python2print("=== Falsy Values ===")3print(f"bool(False): {bool(False)}")4print(f"bool(0): {bool(0)}")5print(f"bool(0.0): {bool(0.0)}")6print(f"bool(''): {bool('')}")7print(f"bool([]): {bool([])}")8print(f"bool(None): {bool(None)}")910# Truthy values11print("\n=== Truthy Values ===")12print(f"bool(True): {bool(True)}")13print(f"bool(1): {bool(1)}")14print(f"bool(-1): {bool(-1)}")15print(f"bool('hi'): {bool('hi')}")16print(f"bool([1, 2]): {bool([1, 2])}")1718# Practical use in conditions19name→ (empty) = ""20if name:
    output=== Falsy Values ===
bool(False): False
bool(0): False
bool(0.0): False
bool(''): False
bool([]): False
bool(None): False

=== Truthy Values ===
bool(True): True
bool(1): True
bool(-1): True
bool('hi'): True
bool([1, 2]): True

  2. else:

    20if name:21    print(f"Hello, {name}!")22else:23    print("Name is empty")
    outputName is empty

  3. items ← [1, 2, 3]

    25items→ [1, 2, 3] = [1, 2, 3]26if items:

  4. if items:

    25items = [1, 2, 3]26if items[1, 2, 3]:27    print(f"Has {len(items[1, 2, 3])} items")28else:
    outputHas 3 items
bool() Convert to boolean: `bool(0)` → `False`, `bool(1)` → `True`
truthy/falsy Falsy: `False`, `0`, `""`, `[]`, `None`. Everything else is truthy.

Exercise: parse_errors.py

Handle invalid input: what happens with ValueError?