How to check the size of a float in python?

Question

I want to check whether a float is actually 32 or 64bits (and the number of bits of a numpy float array). There should be a built-in, but just didn\'t find out...

Answer 1

python -c "import sys,math; print(sys.float_info.mant_dig + math.ceil(math.log2(sys.float_info.max_10_exp - sys.float_info.min_10_exp)) + 1)"

"+1" because one digit in the mantissa is stored implicitly.

Answer 2

numpy.finfo lists sizes and other attributes of float32 ..., including
nexp : number of bits in the exponent including its sign and bias.
nmant : number of bits in the mantissa.
On a machine with IEEE-754 standard floating point,

import numpy as np
for f in (np.float32, np.float64, float):
    finfo = np.finfo(f)
    print finfo.dtype, finfo.nexp, finfo.nmant

will print e.g.

float32 8 23
float64 11 52
float64 11 52

(Try float16 and float128 too.)

Answer 3

Properties of a Python float can be requested via sys.float_info. It returns information such as max/min value, max/min exp value, etc. These properties can potentially be used to calculate the byte size of a float. I never encountered anything else than 64 bit, though, on many different architectures.

The items of a NumPy array might have different size, but you can check their size in bytes by a.itemsize, where a is a NumPy array.

Answer 4

The range of floating point values is available in the sys.float_info object.

As Sven says, for CPython float is always 64-bit. But Python's language reference says

You are at the mercy of the underlying machine architecture (and C or Java implementation) for the accepted range ...".

So this is not necessarily the case for other Python implementations.

Answer 5

print numpy.finfo(numpy.float)
Machine parameters for float64
---------------------------------------------------------------------
precision= 15   resolution= 1.0000000000000001e-15
machep=   -52   eps=        2.2204460492503131e-16
negep =   -53   epsneg=     1.1102230246251565e-16   
minexp= -1022   tiny=       2.2250738585072014e-308
maxexp=  1024   max=        1.7976931348623157e+308
nexp  =    11   min=        -max
---------------------------------------------------------------------