miscalculation on "100*e-6"

[DeaD_EyE]

The floating-point is a representation in binary system, which cannot represent all decimals.
It's mathematically proofed.

Decimal could help, but keep in mind, that it's much slower than floating-point arithmetic.

from decimal import Decimal

print(Decimal(100) * Decimal("1e-6"))
print(Decimal(100e6) * Decimal("1e-6"))

Output:
Decimal('0.000100')
Decimal('100.000000')

If you need floating-point with higher precision and fast, then try mpmath.
This will not eliminate the error, but you can get closer to a result.