struct.unpack(struct.pack(float)) has roundoff error?

Question

When testing my library, Construct , I found out that tests fail when numbers are built then parsed back to a float. Should floats not be represented exactly as in-memory floats?

In [14]: d = struct.Struct("<f")

In [15]: d.unpack(d.pack(1.23))
Out[15]: (1.2300000190734863,)

Answer 1

Floating point is inherently imprecise, but you are packing a double-precision float ( binary64 ) into a single-precision ( binary32 ) space there. See Basic and interchange formats in the Wikipedia article on IEEE floating point formats; the Python float format uses double precision (see the standard types docs ; Floating point numbers are usually implemented using double in C ).

Use d to use double precision:

>>> import struct
>>> d = struct.Struct("<d")
>>> d.unpack(d.pack(1.23))
(1.23,)

From the Format characters section :

format: f , C Type: float , Python type: float , Standard size: 4 , Footnote: (5)
format: d , C Type: double , Python type: float , Standard size: 8 , Footnote: (5)

5. For the 'f' and 'd' conversion codes, the packed representation uses the IEEE 754 binary32 (for 'f' ) or binary64 (for 'd' ) format, regardless of the floating-point format used by the platform.