fmod and % have a different rule about the sign of the returned value.

This is documented both in the language reference and in the library

reference, in the appropriate spots.

http://python.org/doc/current/ref/binary.html#l2h-411
The % (modulo) operator yields the remainder from the division of

the first argument by the second. The numeric arguments are first

converted to a common type. A zero right argument raises the

ZeroDivisionError exception. The arguments may be floating point

numbers, e.g., 3.14%0.7 equals 0.34 (since 3.14 equals 4*0.7 + 0.34.)

The modulo operator always yields a result with the same sign as its

second operand (or zero); the absolute value of the result is strictly

smaller than the absolute value of the second operand

[Footnote: While abs(x%y) < abs(y) is true mathematically, for

floats it may not be true numerically due to roundoff. For example,

and assuming a platform on which a Python float is an IEEE 754

double-precision number, in order that -1e-100 % 1e100 have the

same sign as 1e100, the computed result is -1e-100 + 1e100, which

is numerically exactly equal to 1e100. Function fmod() in the math

module returns a result whose sign matches the sign of the first

argument instead, and so returns -1e-100 in this case. Which approach

is more appropriate depends on the application.]

http://python.org/doc/current/lib/mo....html#l2h-1105
fmod(x, y)

Return fmod(x, y), as defined by the platform C library. Note that

the Python expression x % y may not return the same result.

Jeff