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 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 1

Oh, the many, many ways...

String concatenation:

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

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Conversion Specifier:

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

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Using local variable names:

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

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Using str.format():

plot.savefig('hanning{0}.pdf'.format(num)) # Note: This is the preferred way since 3.6 

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Using f-strings:

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

This is the new preferred way:

• PEP-502
• RealPython
• PEP-536

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Using string.Template:

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

Answer 2

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