CODEWITHSUNDEEP
pythondecimalprecisionfloating-accuracy
Zak Stucke
Zak Stucke

Reputation: 452

Identify actual precision of a float

I am interacting with an API that returns floats. I am trying to calculate the number of decimal places with which the API created these floats.

For example:

# API returns the following floats.
>> 0.0194360600000000015297185740.....
>> 0.0193793800000000016048318230.....
>> 0.0193793699999999999294963970.....

# Quite clearly these are supposed to represent:
>> 0.01943606
>> 0.01937938
>> 0.01937937

# And are therefore ACTUALLY accurate to only 8 decimal places.

How can I identify that the floats are actually accurate to 8 decimal places? Once I do that, I can initialize a decimal.Decimal instance with the "true" values rather than the inaccurate floats.

Edit: The number of accurate decimal places returned by the API varies and is not always 8!

Upvotes: 2

Views: 190

Answers (1)

ndmeiri
ndmeiri

Reputation: 5039

If you are using Python 2.7 or Python 3.1+, consider using the repr() builtin.

Here's how it works with your examples in a Python 3.6 interpreter.

>>> repr(0.0194360600000000015297185740)
'0.01943606'
>>> repr(0.0193793800000000016048318230)
'0.01937938'
>>> repr(0.0193793699999999999294963970)
'0.01937937'

This works because repr() shows the minimum precision of the number, n, that still satisfies float(repr(n)) == n.

Given the string representation returned by repr(), you can count the number of digits to the right of the decimal point.

Upvotes: 6

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