How can I use the value of a variable in Python format string

Question

Please tell me something like this is possible in Python. I can't seem to get it to work

MY_LENGTH_CONSTRAINT = 24
myFormatStr = '{mykey:<${MY_LENGTH_CONSTRAINT}s}'
myStr = myFormatStr.format(mykey='Something')

I keep getting

KeyError: 'MY_LENGTH_CONSTRAINT'

unutbu · Accepted Answer · 2012-01-19 22:53:48Z

4

Add mcl = MY_LENGTH_CONSTRAINT to the parameters fed to format:

MY_LENGTH_CONSTRAINT = 24
myFormatStr = '{mykey:<{mlc}s}'
myStr = myFormatStr.format(mykey='Something',
                           mlc = MY_LENGTH_CONSTRAINT)
print(myStr)
# Something

You can also refer to local variables in your format string, and inform format of the values by passing it **locals():

MY_LENGTH_CONSTRAINT = 24
myFormatStr = '{mykey:<{MY_LENGTH_CONSTRAINT}s}'
myStr = myFormatStr.format(mykey='Something', **locals())
print(myStr)
# Something

(or similarly, use global variables, and pass format **globals().)

edited Jan 19, 2012 at 22:53

answered Jan 19, 2012 at 21:35

unutbu

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1 Comment

user1159503 Over a year ago

Of course, but I was wondering if this could be done implicitly. I believe ruby has a feature like the one I'm looking for here in Python.

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