str.format() / f-strings / str.zfill()

# Embed values using the format() method.
"template {}".format(value)
"template {name}".format(name=value)
"template {format_spec}".format(value)

# Embed variables directly using f-strings (Python 3.6+).
f"text {variable} text"
f"text {expression:.2f} text"

# Pad a number with leading zeros.
string.zfill(width)

# Align a string within a specified width.
string.center(width, fillchar=' ')
string.ljust(width, fillchar=' ')
string.rjust(width, fillchar=' ')

# Basic usage of f-strings (the most modern approach).
name = "Taro"
age = 20
print(f"Name: {name}, Age: {age}")  # Name: Taro, Age: 20

# Evaluate an expression inside an f-string.
price = 1980
tax_rate = 0.1
print(f"Price with tax: {price * (1 + tax_rate):.0f}")  # Price with tax: 2178

# Format numbers with format specifiers.
pi = 3.14159265
print(f"Pi: {pi:.2f}")         # Pi: 3.14 (2 decimal places)
print(f"Pi: {pi:.4f}")         # Pi: 3.1416
print(f"Amount: {1000000:,}")  # Amount: 1,000,000 (with comma separator)
print(f"Ratio: {0.856:.1%}")   # Ratio: 85.6% (percentage notation)

# Format a number in different bases.
n = 255
print(f"Decimal: {n:d}, Hex: {n:x}, Binary: {n:b}, Octal: {n:o}")
# Decimal: 255, Hex: ff, Binary: 11111111, Octal: 377

# Align strings within a field width.
print(f"{'left':<10}|")    # left      |
print(f"{'center':^10}|")  #   center  |
print(f"{'right':>10}|")   #      right|

# Using format() for compatibility with Python 3.5 and earlier.
print("Name: {}, Age: {}".format(name, age))
print("Name: {name}, Age: {age}".format(name="Hanako", age=25))
print("{0} and {1} and {0}".format("A", "B"))  # A and B and A

# Applying format specifiers with format().
print("Amount: {:,}".format(1234567))   # Amount: 1,234,567
print("Ratio: {:.1%}".format(0.856))    # Ratio: 85.6%

# Zero-padding with zfill().
order_no = "42"
print(order_no.zfill(6))  # 000042 (pads with zeros to 6 digits)

# Zero-pad month and day values.
month = 3
day = 5
print(f"{month:02d}/{day:02d}")  # 03/05

# Aligning text with center(), ljust(), and rjust().
label = "PYTHON"
print(label.center(20, "="))  # =======PYTHON=======
print(label.ljust(20, "-"))   # PYTHON--------------
print(label.rjust(20, "-"))   # --------------PYTHON

Python offers three main ways to format strings: f-strings, format(), and the % operator. In modern Python (3.6+), f-strings are recommended because they are the most readable and the fastest. Inside an f-string, you can write not only variables but also arbitrary expressions and function calls.

The format specifier syntax follows the pattern :flags width.precision type. Learning the most commonly used specifiers will make your code much more concise. To display a floating-point number as an integer, use {:.0f} (zero decimal places). Using {:d} on a float will raise a TypeError.

When debugging, you can write f"{variable_name=}" (Python 3.8+) to print both the variable name and its value at once. For example, f"{pi=}" outputs pi=3.14159265. For numeric type conversions, see int() / float() / str().