I couldn’t find anything about it in the Python style guide, as you said, but searching for “unary operator spacing” brought more hits from various languages, such as this for Javascript:

No space should separate a unary operator and its operand except when the operator is a word such as typeof.

Or this for C:

Don’t use spaces around unary operators, except sizeof and casts

In the Linux kernel:

Use one space around (on each side of) most binary and ternary operators, […] but no space after unary operators:

Note that a writing style for code, which is usually written with a fixed-width font and mostly Latin characters, is quite different than that for equations, where you can find both latin and greek letters, topological constructs such as fractions or those marks surrounding that epsilon you have there, and other things that make them a whole lot more complex than code.

I’ve run across this before, and I used

var false_positive = 0 - true_positive

to make sure the intent is clear. That is the more readable form.

I think the second one is marginally less bad, because the minus sign is a little less likely to be read as part of the word; However, I think both forms are bad.

I would replace the whole thing with

false_positive = -1 * true_positive

or even

false_positive = (-1) * true_positive

in any language.