I don’t understand what %s and %d do and how they work.

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 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

The %d and %s string formatting “commands” are used to format strings. The %d is for numbers, and %s is for strings.

For an example:

print("%s" % "hi")

and

print("%d" % 34.6)

To pass multiple arguments:

print("%s %s %s%d" % ("hi", "there", "user", 123456)) will return hi there user123456

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'

%d and %s are placeholders, they work as a replaceable variable. For example, if you create 2 variables

variable_one = "Stackoverflow"
variable_two = 45

you can assign those variables to a sentence in a string using a tuple of the variables.

variable_3 = "I was searching for an answer in %s and found more than %d answers to my question"

Note that %s works for String and %d work for numerical or decimal variables.

if you print variable_3 it would look like this

print(variable_3 % (variable_one, variable_two))

I was searching for an answer in StackOverflow and found more than 45 answers to my question.

They are format specifiers. They are used when you want to include the value of your Python expressions into strings, with a specific format enforced.

See Dive into Python for a relatively detailed introduction.

As per latest standards, this is how it should be done.

print("My name is {!s} and my number is{:d}".format("Agnel Vishal",100))

Do check python3.6 docs and sample program

In case you would like to avoid %s or %d then..

name = 'marcog'
number = 42
print ('my name is',name,'and my age is:', number)

Output:

my name is marcog and my name is 42

%s is used to hold space for string %d is used to hold space for number

name = "Moses";
age = 23
print("My name is %s am CEO at MoTech Computers " %name)
print("Current am %d years old" %age)
print("So Am %s and am %d years old" %(name,age))

Program output

this video goes deep about that tip https://www.youtube.com/watch?v=4zN5YsuiqMA

speaking of which …
python3.6 comes with f-strings which makes things much easier in formatting!
now if your python version is greater than 3.6 you can format your strings with these available methods:

name = "python"

print ("i code with %s" %name)          # with help of older method
print ("i code with {0}".format(name))  # with help of format
print (f"i code with {name}")           # with help of f-strings