Why are commas not needed for modulo string formatting when printing?

It is, as you said, an operator so it doesn't make any sense to place a comma somewhere between the operator and the two operands. The first operand is the template string, and the second operand is the tuple with the values to format into the template string. The print() function gets one argument — the result of the formatting operation.

Maybe it helps to bind both operands to names, so the expression becomes a bit simpler:

animal = "giraffe"
age = 25
template = "A %s can live up to %d years"
values = (animal, age)
print(template % values)

The % operator is syntactically no different than other binary operators like + or / and so on.

posted about 2 years ago

CC BY-SA 4.0

2y ago

__blackjack__‭

141 reputation 0 3 14 5

The modulo operator is a binary (2 argument) operator which returns a single value, and can be used for both numeric calculations and strings:

x = 5 % 2
print(x) # prints "1"

y = "hello %s" % 3
print(y) # prints "hello 3"

When you use a modulo operator for string substitution, you are creating a single string as the result. In your example, you then pass this created string directly to print() as a single argument. There is no need for a comma because commas are used to separate arguments, and you have only a single argument: the result of the string substitution. The substitution is not being performed by print(), but by the modulo operator itself, as you can see if you construct the string in an interactive Python session without ever calling print():

>>> animal = "giraffe"
>>> age = 25
>>> "A %s can live up to %d years" %(animal,age)
'A giraffe can live up to 25 years'

To put it another way, you don't put a comma before the modulo operator for the same reason you don't put a comma before the plus operator (or any other kind of 2-argument operator):

print(2 + 2) # single argument to print() with the value 4

String formatting

Although it doesn't directly relate to your specific question, be aware that using the modulo operator for string formatting is no longer recommended for new Python code.

You should either use the format() method:

print("A {} can live up to {} years".format(animal, age))

or in Python 3.6 or above, the even more readable f-strings:

print(f"A {animal} can live up to {age} years")

% is one of the oldest Python syntaxes for interpolating strings. It basically works like:

template_string % data_tuple

In fact, you can literally do that:

>>> s = "%d %d"
>>> t = (1, 2)
>>> s % t
'1 2'

template_string is the template, data_tuple is the values to be used for rendering the template. Obviously they must match in number - if you have too few values in your tuple you'll get an error.

These days, there are much better ways to do the same thing and I would recommend avoiding %:

• "A {} can live up to {} years".format(animal, age)" - looks like a plain old function call, like "HeLLo".lower().
• f"A {animal} can live up to {age}" ("f-string") - syntactic sugar to make the above call to .format easier.

posted almost 2 years ago

CC BY-SA 4.0

matthewsnyder‭

2277 reputation 52 61 267 93