How do I put a variable’s value inside a string?

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?

Oh, the many, many ways…

Using f-strings:

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

This was added in 3.6 and is the new preferred way.

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Using str.format():

plot.savefig('hanning{0}.pdf'.format(num))

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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 (neat trick):

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

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Using string.Template:

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

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See also:

• Fancier Output Formatting – The Python Tutorial
• Python 3’s f-Strings: An Improved String Formatting Syntax (Guide) – RealPython

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')

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:

printf-style String Formatting

You can use + as the normal string concatenation function as well as str().

"hello " + str(10) + " world" == "hello 10 world"

In general, you can create strings using:

stringExample = "someString " + str(someNumber)
print(stringExample)
plot.savefig(stringExample)

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.

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)]

You can make dict and substitute variables in your string.

var = {"name": "Abdul Jalil", "age": 22}
temp_string = "My name is %(name)s. I am %(age)s years old." % var

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

str(num)