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Q&A

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

+4
−1

I'm still a novice in Python. But have come across a question about format specifiers. For example, if I have two variables that are called `animal` and `age`. And print a string in the console using format specifiers. Why do we place the modulo operator `%(animal,age)` next to the string without a comma? Does Python automatically detect that it needs two parameters to execute?

Here is the code example:

``````animal = "giraffe"
age = 25

print("A %s can live up to %d years" %(animal,age))
``````
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+4
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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.

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+3
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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")``
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