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pythonstringformatting

How to dynamically format string representation of float number in python?

Hi I would like to dynamically adjust the displayed decimal places of a string representation of a floating point number, but i couldn't find any information on how to do it.

E.g:

precision = 8

n = 7.12345678911

str_n = '{0:.{precision}}'.format(n)

print(str_n) should display -> 7.12345678

But instead i'm getting a "KeyError". What am i missing?

Solution

  • You need to specify where precision in your format string comes from:

    precision = 8
n = 7.12345678911
print('{0:.{precision}}'.format(n, precision=precision))

    The first time, you specified which argument you'd like to be the number using an index ({0}), so the formatting function knows where to get the argument from, but when you specify a placeholder by some key, you have to explicitly specify that key.

    It's a little unusual to mix these two systems, i'd recommend staying with one:

    print('{number:.{precision}}'.format(number=n, precision=precision))  # most readable
print('{0:.{1}}'.format(n, precision))
print('{:.{}}'.format(n, precision))  # automatic indexing, least obvious

    It is notable that these precision values will include the numbers before the point, so 

    >>> f"{123.45:.3}"
'1.23e+02'

    will give drop drop the decimals and only give the first three digits of the number. Instead, the f can be supplied to the type of the format (See the documentation) to get fixed-point formatting with precision decimal digits.

    print('{number:.{precision}f}'.format(number=n, precision=precision))  # most readable
print('{0:.{1}f}'.format(n, precision))
print('{:.{}f}'.format(n, precision))  # automatic indexing, least obvious