What's the difference between %s and %d in Python string formatting?

Question

They are used for formatting strings. %s acts a placeholder for a string while %d acts as a placeholder for a number. Their associated values are passed in via a tuple using the % operator.

name = 'marcog'
number = 42
print '%s %d' % (name, number)

will print marcog 42. Note that name is a string (%s) and number is an integer (%d for decimal).

See https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting for details.

In Python 3 the example would be:

print('%s %d' % (name, number))

from python 3 doc

%d is for decimal integer

%s is for generic string or object and in case of object, it will be converted to string

Consider the following code

name ='giacomo'
number = 4.3
print('%s %s %d %f %g' % (name, number, number, number, number))

the out put will be

giacomo 4.3 4 4.300000 4.3

as you can see %d will truncate to integer, %s will maintain formatting, %f will print as float and %g is used for generic number

obviously

print('%d' % (name))

will generate an exception; you cannot convert string to number

%s is used as a placeholder for string values you want to inject into a formatted string.

%d is used as a placeholder for numeric or decimal values.

For example (for python 3)

print ('%s is %d years old' % ('Joe', 42))

Would output

Joe is 42 years old

These are all informative answers, but none are quite getting at the core of what the difference is between %s and %d.

%s tells the formatter to call the str() function on the argument and since we are coercing to a string by definition, %s is essentially just performing str(arg).

%d on the other hand, is calling int() on the argument before calling str(), like str(int(arg)), This will cause int coercion as well as str coercion.

For example, I can convert a hex value to decimal,

>>> '%d' % 0x15
'21'

or truncate a float.

>>> '%d' % 34.5
'34'

But the operation will raise an exception if the argument isn't a number.

>>> '%d' % 'thirteen'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: %d format: a number is required, not str

So if the intent is just to call str(arg), then %s is sufficient, but if you need extra formatting (like formatting float decimal places) or other coercion, then the other format symbols are needed.

With the f-string notation, when you leave the formatter out, the default is str.

>>> a = 1
>>> f'{a}'
'1'
>>> f'{a:d}'
'1'
>>> a = '1'
>>> f'{a:d}'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Unknown format code 'd' for object of type 'str'

The same is true with string.format; the default is str.

>>> a = 1
>>> '{}'.format(a)
'1'
>>> '{!s}'.format(a)
'1'
>>> '{:d}'.format(a)
'1'

These are placeholders:

For example: 'Hi %s I have %d donuts' %('Alice', 42)

This line of code will substitute %s with Alice (str) and %d with 42.

Output: 'Hi Alice I have 42 donuts'

This could be achieved with a "+" most of the time. To gain a deeper understanding to your question, you may want to check {} / .format() as well. Here is one example: Python string formatting: % vs. .format

also see here a google python tutorial video @ 40', it has some explanations https://www.youtube.com/watch?v=tKTZoB2Vjuk