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?

9 Answers

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 preferred way since 3.6 

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

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:

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') 

Not sure exactly what all the code you posted does, but to answer the question posed in the title, you can use + as the normal string concat function as well as str().

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

Hope that helps!

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)