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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")

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](https://docs.python.org/3/reference/lexical_analysis.html#f-strings):
print(f"A {animal} can live up to {age} years")

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