Python Core | VoidX Academy

23. Python String Formatting

Data Processing

Python String Formatting

To make sure a string displays exactly as expected, we format it. String formatting allows you to inject variables into strings, control decimal precision, align text, and pad numbers with zeros.

While Python has older methods like the % operator and .format(), the modern, standard, and most readable way to format strings in Python 3.6+ is using f-Strings (Formatted String Literals).

F-Strings Refresher

To specify a string as an f-string, simply put an f (or F) immediately in front of the opening quotation mark. Then, you can place variables directly inside curly braces {}.

name = "Jane"
age = 28

print(f"My name is {name} and I am {age} years old.")

OutputMy name is Jane and I am 28 years old.

Evaluating Expressions

F-strings do more than just insert variables; they can evaluate full Python expressions (math, functions, methods) directly inside the curly braces.

price = 50
tax = 0.20

# Doing math inside the f-string
print(f"The total price is ${price + (price * tax)}")

# Calling methods inside the f-string
user = "admin"
print(f"Welcome back, {user.upper()}!")

OutputThe total price is $60.0 Welcome back, ADMIN!

Formatting Numbers (Decimals)

When working with floats (especially currency), you often want to restrict the number of decimal places. You can add a format specifier by adding a colon : inside the curly braces, followed by the format code.

price = 49

# .2f means "fixed point number with 2 decimal places"
print(f"The price is ${price:.2f}")

pi = 3.14159265
print(f"Pi rounded to 3 places: {pi:.3f}")

OutputThe price is $49.00 Pi rounded to 3 places: 3.142

Formatting Large Numbers (Commas)

Large numbers are difficult to read without separators. You can use the comma , format specifier to automatically insert thousand separators.

population = 8400000000
revenue = 1543200.50

# Insert commas for readability
print(f"Earth Population: {population:,}")

# Chain comma and decimal specifiers together
print(f"Company Revenue: ${revenue:,.2f}")

OutputEarth Population: 8,400,000,000 Company Revenue: $1,543,200.50

Formatting Percentages

If you have a decimal ratio that you want to display as a percentage, use the % format specifier. It multiplies the number by 100 and adds a percent sign automatically.

success_rate = 0.875

# The .1% means "convert to percentage and show 1 decimal place"
print(f"The system uptime is {success_rate:.1%}")

OutputThe system uptime is 87.5%

Padding and Alignment

When printing data in tables or columns, you need text to line up perfectly. You can specify a minimum width and an alignment direction.

:< Left aligns the result (within the available space)
:> Right aligns the result
:^ Center aligns the result

item1 = "Apples"
item2 = "Watermelons"

# Reserve exactly 15 spaces for the text, left aligned
print(f"|{item1:<15}| $2.00")
print(f"|{item2:<15}| $5.00")

# Center align within 15 spaces
print(f"|{item1:^15}|")

# Right align within 15 spaces
print(f"|{item1:>15}|")

Padding with Zeros

Sometimes IDs, invoice numbers, or binary strings need to be padded with leading zeros to maintain a consistent length. Use the 0 flag before the width number.

invoice_number = 42

# Pad with zeros to make it exactly 5 digits long
print(f"Invoice: #{invoice_number:05}")

binary_val = 5

# Format as binary (b) and pad with zeros to 8 bits
print(f"Binary: {binary_val:08b}")

OutputInvoice: #00042 Binary: 00000101

💡Best Practice: While older Python codebases will use .format() or the %s syntax, you should exclusively write f-strings in new code. They evaluate faster at runtime and are significantly easier for humans to read and maintain.

23. Python String Formatting

Data Processing

Python String Formatting

To make sure a string displays exactly as expected, we format it. String formatting allows you to inject variables into strings, control decimal precision, align text, and pad numbers with zeros.

While Python has older methods like the % operator and .format(), the modern, standard, and most readable way to format strings in Python 3.6+ is using f-Strings (Formatted String Literals).

F-Strings Refresher

To specify a string as an f-string, simply put an f (or F) immediately in front of the opening quotation mark. Then, you can place variables directly inside curly braces {}.

name = "Jane"
age = 28

print(f"My name is {name} and I am {age} years old.")

OutputMy name is Jane and I am 28 years old.

Evaluating Expressions

F-strings do more than just insert variables; they can evaluate full Python expressions (math, functions, methods) directly inside the curly braces.

price = 50
tax = 0.20

# Doing math inside the f-string
print(f"The total price is ${price + (price * tax)}")

# Calling methods inside the f-string
user = "admin"
print(f"Welcome back, {user.upper()}!")

OutputThe total price is $60.0 Welcome back, ADMIN!

Formatting Numbers (Decimals)

price = 49

# .2f means "fixed point number with 2 decimal places"
print(f"The price is ${price:.2f}")

pi = 3.14159265
print(f"Pi rounded to 3 places: {pi:.3f}")

OutputThe price is $49.00 Pi rounded to 3 places: 3.142

Formatting Large Numbers (Commas)

Large numbers are difficult to read without separators. You can use the comma , format specifier to automatically insert thousand separators.

population = 8400000000
revenue = 1543200.50

# Insert commas for readability
print(f"Earth Population: {population:,}")

# Chain comma and decimal specifiers together
print(f"Company Revenue: ${revenue:,.2f}")

OutputEarth Population: 8,400,000,000 Company Revenue: $1,543,200.50

Formatting Percentages

If you have a decimal ratio that you want to display as a percentage, use the % format specifier. It multiplies the number by 100 and adds a percent sign automatically.

success_rate = 0.875

# The .1% means "convert to percentage and show 1 decimal place"
print(f"The system uptime is {success_rate:.1%}")

OutputThe system uptime is 87.5%

Padding and Alignment

When printing data in tables or columns, you need text to line up perfectly. You can specify a minimum width and an alignment direction.

:< Left aligns the result (within the available space)
:> Right aligns the result
:^ Center aligns the result

item1 = "Apples"
item2 = "Watermelons"

# Reserve exactly 15 spaces for the text, left aligned
print(f"|{item1:<15}| $2.00")
print(f"|{item2:<15}| $5.00")

# Center align within 15 spaces
print(f"|{item1:^15}|")

# Right align within 15 spaces
print(f"|{item1:>15}|")

Padding with Zeros

Sometimes IDs, invoice numbers, or binary strings need to be padded with leading zeros to maintain a consistent length. Use the 0 flag before the width number.

invoice_number = 42

# Pad with zeros to make it exactly 5 digits long
print(f"Invoice: #{invoice_number:05}")

binary_val = 5

# Format as binary (b) and pad with zeros to 8 bits
print(f"Binary: {binary_val:08b}")

OutputInvoice: #00042 Binary: 00000101

23. Python String Formatting

Python String Formatting

F-Strings Refresher

Evaluating Expressions

Formatting Numbers (Decimals)

Formatting Large Numbers (Commas)

Formatting Percentages

Padding and Alignment

Padding with Zeros

Knowledge Check

23. Python String Formatting

Python String Formatting

F-Strings Refresher

Evaluating Expressions

Formatting Numbers (Decimals)

Formatting Large Numbers (Commas)

Formatting Percentages

Padding and Alignment

Padding with Zeros

Knowledge Check