Assigning intermediate value to a variable makes the computation result complex

Question

Python gives me a different result if I assign one of the intermediate steps to a variable, like this:

>>> -0.207 ** 0.66 - 1
-1.3536229379434348
>>> a = -0.207
>>> a ** 0.66 - 1
(-1.1703591496008927+0.30988214273656856j)

For this simple calculation, if I assign -0.207 to a temporary variable a, then the result of a ** 0.66 - 1 evaluates to a complex number.

Why does this happen, and how do I stop Python from doing that?

Answer 1

The correct answer to your statement (according to python) is a negative number. -0.207**0.66-1 evaluates to about -1.35.
The reason this is happening is that you're miscalculating in the one-liner: -0.207 ** 0.66 - 1 actually evaluates to -(0.207 ** 0.66) - 1 and not to (-0.207) ** 0.66 - 1 like you'd expect.

When you separate the lines, you're changing the calculation to the second statement.

To stop this from happening, use explicit parentheses where there might be any ambiguity.