How do I put a variable inside a string?

python string variables
Gish picture Gish · Jun 2, 2010 · Viewed 759.8k times · Source

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 line

I have to run the program for several different numbers, so I'd like to do a loop. But inserting the variable like this doesn't work:

plot.savefig('hanning', num, '.pdf')

How do I insert a variable into a Python string?

Answer

Dan McDougall picture Dan McDougall · Jun 3, 2010

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()))

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