How do I put a variable inside a string?

Question

I would like to put an int into a string. This is what I am doing at the moment:

num = 40
plot.savefig(\'hanning40.pdf\') #problem          


        

Answer 1

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')

Answer 2

I had a need for an extended version of this: instead of embedding a single number in a string, I needed to generate a series of file names of the form 'file1.pdf', 'file2.pdf' etc. This is how it worked:

['file' + str(i) + '.pdf' for i in range(1,4)]

Answer 3

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

Answer 4

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()))

Answer 5

You just have to cast the num varriable into a string using

str(num)

Answer 6

If you would want to put multiple values into the string you could make use of format

nums = [1,2,3]
plot.savefig('hanning{0}{1}{2}.pdf'.format(*nums))

Would result in the string hanning123.pdf. This can be done with any array.