How do I put a variable inside a string?

Oh, the many, many ways...

String concatenation:

plot.savefig('hanning' + str(num) + '.pdf')

Conversion Specifier:

plot.savefig('hanning%s.pdf' % num)

Using local variable names:

plot.savefig('hanning%(num)s.pdf' % locals()) # Neat trick

Using str.format():

plot.savefig('hanning{0}.pdf'.format(num)) # Note: This is the new preferred way

Using f-strings:

plot.savefig(f'hanning{num}.pdf') # added in Python 3.6

Using string.Template:

plot.savefig(string.Template('hanning${num}.pdf').substitute(locals()))

plot.savefig('hanning(%d).pdf' % num)

The % operator, when following a string, allows you to insert values into that string via format codes (the %d in this case). For more details, see the Python documentation:

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

With the introduction of formatted string literals ("f-strings" for short) in Python 3.6, it is now possible to write this with a briefer syntax:

>>> name = "Fred"
>>> f"He said his name is {name}."
'He said his name is Fred.'

With the example given in the question, it would look like this

plot.savefig(f'hanning{num}.pdf')