What does the %d specifier mean in Python?

The specifier %d is used in string formatting operations (with the % operator) and in the str.format() method to indicate the position where a decimal integer will be inserted.

When using % formatting:

# Formatting with %d
age = 30
formatted_string = "He is %d years old." % age
# formatted_string will be "He is 30 years old."

When using str.format():

# Formatting with str.format() and positional placeholder
age = 30
formatted_string = "He is {} years old.".format(age)
# formatted_string will be "He is 30 years old."

# Formatting with str.format() and named placeholder
age = 30
formatted_string = "He is {years} years old.".format(years=age)
# formatted_string will be "He is 30 years old."

Unlike % formatting, str.format() does not use an explicit %d, but determines the data type based on the value passed. However, conceptually, the placeholder {} or {name} in this context performs the same role as %d — inserting an integer. You can also explicitly specify the type in str.format() using format specifiers inside curly braces, for example:

# Explicitly specifying 'd' type in str.format() for decimal integer
age = 30
formatted_string = "He is {:d} years old.".format(age)
# formatted_string will be "He is 30 years old."

Thus, %d directly (in % formatting) or indirectly (via the :d specifier in str.format()) means "decimal integer".