Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Welcome to Software Development on Codidact!

Will you help us build our independent community of developers helping developers? We're small and trying to grow. We welcome questions about all aspects of software development, from design to code to QA and more. Got questions? Got answers? Got code you'd like someone to review? Please join us.

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

Parent

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

+6
−3

Suppose I have two variables that are called animal and age, and print them as a string in the console like so:

animal = "giraffe"
age = 25

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

Why shouldn't there be a comma between the string and the %(animal,age) part? Does Python automatically detect that it needs two parameters to execute?

History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.
Why should this post be closed?

1 comment thread

As a side note, the documentation calls it ["Old string formatting"](https://docs.python.org/3/tutori... (1 comment)
Post
+6
−0

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")
History
Why does this post require attention from curators or moderators?
You might want to add some details to your flag.

1 comment thread

Works for me (1 comment)
Works for me
Bennshinpoes‭ wrote over 1 year ago

Very good explained, and gives me good advice for using the modulo operator.