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?

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

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()))

Method 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

Method 3

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')

Method 4

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!

Method 5

In general, you can create strings using:

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

Method 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.

Method 7

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)]

Method 8

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

Method 9

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

str(num)

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0