格式化值以浮动和宽度和精度浮动

Question

I would like to validate a string input to verify if it could be formated to a valid float for given width and precision.

width = 10
precision = 4

value = '12'

try:
   "{10.4f}".format(value)
except:
   print "not valid"

this test fails but it should works because 12 could be considered as a float 12.0000

I would also like to have a dynamic test because width and precision are variables.

Thank you for your help!

Answer 1

You forgot the : colon:

"{:10.4f}".format(float(value))

otherwise Python interprets the first digit as a positional parameter index.

Each parameter can be set with it's own placeholder:

"{value:{width}.{precision}f}".format(
    value=float(value), width=width, precision=precision)

width and precision arguments are interpolated before the value is formatted.

This is, however, not a good test for floating point inputs. The float value 12.234 cannot be exactly represented; binary fractions can only approximate it:

>>> format(12.234, '.53f')
'12.23399999999999998578914528479799628257751464843750000'

so this value wouldn't 'fit' your 10.4 constraints, yet look when rounded like a perfectly valid input to give.

Any floating point value can be formatted to a fixed width, in any case:

>>> format(10**11 + 0.1, '10.4f')
'100000000000.1000'

No ValueError will be raised; the 10 in the width parameter means: produce a string that is at least this many characters wide, pad with spaces if it is shorter , and this width includes the decimal point and the decimals.

To validate floating point input, the best you can do is test that it can be converted to a float, and then test for mininum and maxmimum values:

try:
    value = float(value)
except ValueError:
    # cannot be converted to a valid float
    return "Not a valid input"
else:
    if 0 <= value < 10 ** 11:
        return "Value out of range"

Answer 2

Let's use Python 3

Format Specifier f"{var}"

>>> width = 10
>>> precision = 4
>>> value = decimal.Decimal("12.34567")
>>> f"result: {value:{width}.{precision}}"  # nested fields
'result:      12.35'

Answer 3

There is a type mismatch here I think ...

value = 12

not

value = '12'

Answer 4

First, instead of

"{10.4f}".format(value)

write it as: "{10.4f}".format(float(value))

to convert to string to float first

On how to generalize this: instead of having the string "{10.4f}" hard-coded, create it

width = 10
precision = 4
formatting_string = '{' + str(width) + '.' + str(precision) + 'f}'

The, use

formatting_string.format(float(value))

Answer 5

Using string rjust and ljust:

value.rjust(width)+'.'.ljust(precision+1,'0')+'f'

Output:

        12.0000f

In this way, we donot need to convert the value to float