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What Does ‘%s’ mean in Python?

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What does '%s' mean in python?
What does '%s' mean in python?

What does ‘%s’ mean in python? In python, there are many ways to print a statement.

One way of doing this is by using format specifiers.

Format specifiers are special arguments that specify the format of input that has to be taken by the user and the format that has to be given to the user.

%s is a format specifier.

The role of %s is that it tells the python interpreter about what format text it will be printing, on the console. A string is a format in this case.

What does '%s' mean in python?
What does ‘%s’ mean in python?

So the syntax goes something like this.

Let’s say I have a variable called foo and I have the string “this is foo”

So

  1. foo = “this is foo”
  2. #that is the variable that has been initiated
  3.  
  4. #now let’s print it.
  5.  
  6. print(“the data in foo is %s” %(foo))
  7.  
  8. # this is equivalent to
  9.  
  10. print(“the data in foo is “+foo)
  11.  Some other format important specifiers are %d: integer %f : float.

More on What Does ‘%s’ mean in python?

In Python ‘%s’ is simply a string literal consisting of two punctuation characters. By itself the percent sign (%) enclosed in any form of quotes, in Python, is just another character. There’s nothing special about that.

However, ‘%s’ is most often used in conjunction with the % operator for strings. This operator is over-ridden for strings in much the same way that + and * are. If I have an expression ‘foo’ + ‘bar’ it will evaluate into a result of ‘foobar’ (the concatenation of the two string literals). An expression like ‘-‘ * 10 will evaluate into ‘———-‘ (ten hyphens). Notice how concatenation and repetition of strings are analogous to the arithmetic operations for addition and multiplication.

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The % operator is commonly used in programming languages to compute the arithmetic “modulus” (also known as “remainder”). So 3%2 and 10%3 both evaluate to 1 (the remainder of dividing one by the other).

In English there’s a technical usage of the term “modulo” — to wit: (“except for differences accounted for by.”) For instance I might say that I’ve completed a project modulo a list of change orders. This is, roughly analogous to an expression like: ‘Project %s is complete except for %s’ % (project_name, change_list) … where the ‘%s’ “replaceable parameter expressions” (or “tokens”) are points in that string which will be replaced by the string representations of the values associated with those two variables project_name and change_list.

There are a number of replaceable tokens which can appear in the “formatting string” to the right of a % operator. These are documented here: 5. Built-in Types: String formatting. For the most part these are compatible with the formatting features of the printf() family of functions in the C programming language’s standard libraries … though they are more like sprintf() or snprintf() than printf() per se. They evaluate into a new string regardless of whether your code is printing them or not.

As you’ll see there are quite a few nuances to string formatting … they allow you to specify numeric precision and formatting, leading signs and leading zeros, hexadecimal representations with 0x or 0X (upper for lower case) prefixes, and scientific notation with e or E (lower or upper case) for the exponentiation annotation, and so on.

There’s also a special, Python specfic formatting token: %r which is evaluated into the “representation” of a Python object. This is the same way that a Python object is represented in the interactive interpreter and in certain diagnostics output (such as in a stack trace printed by an uncaught exception). It’s mostly used for your own custom logging and diagnostic output. Ideal the %r of a Python object could be eval()-uated into an identical object instantiation; but that’s often not feasible.

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C-styled string formatting can be notoriously tricky and sometimes downright frustrating. It’s sometimes necessary to build up your final string through a chain of multiple formatting expressions to get a more intricate form of output.

Modern versions of Python (from 2.6 and later and in 3.x) support a newer and far more expressive form of string formatting which is accessed via the logically named .format() method on string object.

My earlier example could be rendered as: ‘Project %s is complete except for %s’.format(‘Spam Cooker’, ‘the off switch’) (where I’ve just replaced project_name and change_list with some silly literal strings … the variables would work just as well).

The new style of string formatting is covered quite extensively in the documentation, Common string operations: string formatting – Python 3.4.8 and on a site dedicated to the purpose: pyformat.info: Using % and .format() for great good!

The syntax of the new style formatting is intended to be more similar to modern template rendering engines. Python also supports a simple template rendering feature which was introduced in version 2.4 as documented in Template strings.

There are also a few world-class template rendering and management systems for Python including Jinja2Cheetah3Genshi, and the incredibly high performance spitfire, among many, many others. Most of these are used extensively in web applications frameworks for dynamically generating HTML (and possibly CSS).

So, in conclusion, ‘%s’ is just a string. But it’s a string that’s often going to be used with Python’s old (C-language inspired) string formatting operator. There are many other string formatting tokens (replaceable parameters) and there are a few other options, included in the Python standard libraries and available as extension modules, for doing more sophisticated string formatting and template rendering.

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