Python 3 Float Decimal Points/Precision

Question

I am reading a text file with floating point numbers, all with either 1 or 2 decimal points. I am using float() to convert a line into a float, and raising a ValueError if that fails. I am storing all floats in a list. When printing it out, I'd like to print it out as a 2 decimal places floating point.

Assume I have a text file with the numbers -3,65, 9,17, 1. I read each one, and once I convert them to float and append them to a list. Now in Python 2, calling float(-3.65) returns -3.65. In Python 3 however, float(-3.65) returns-3.6499999999999999` which loses its precision.

I want to print the list of floats, [-3.6499999999999999, 9.1699999999999999, 1.0] with 2 decimal points only. Doing something along the lines of '%.1f' % round(n, 1) would return a string. How can I return a list of all two decimal points of floats, and not strings? So far, I rounded it using [round(num, 2) for num in list] but would need to set the decimal points / precision instead of round().

Answer 1

The comments state the objective is to print to 2 decimal places.

There's a simple answer for Python 3:

>>> num=3.65 >>> "The number is {:.2f}".format(num) 'The number is 3.65' 

or equivalently with f-strings (Python 3.6+):

>>> num = 3.65 >>> f"The number is {num:.2f}" 'The number is 3.65' 

As always, the float value is an approximation:

>>> "{}".format(num) '3.65' >>> "{:.10f}".format(num) '3.6500000000' >>> "{:.20f}".format(num) '3.64999999999999991118' 

I think most use cases will want to work with floats and then only print to a specific precision.

Those that want the numbers themselves to be stored to exactly 2 decimal digits of precision, I suggest use the decimal type. More reading on floating point precision for those that are interested.