understanding negative slice step value [duplicate]

Answer 1

The reason why f[0:5:-1] does not generate any output is because you are starting at 0, and trying to count backwards to 5. This is impossible, so Python returns an empty string.

Instead, you want f[5:0:-1], which returns the string "raboo".

Notice that the string does not contain the f character. To do that, you'd want f[5::-1], which returns the string "raboof".

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You also asked:

I have read start should not pass stop. is that true in case of negative step value also?

No, it's not true. Normally, the start value shouldn't pass the stop value, but only if the step is positive. If the step is negative, then the reverse is true. The start must, by necessity, be higher then the stop value.

Answer 2

You can think like that:

with f[0:5], 0 is the start position and 5-1 the end position.

with f[0:5:-1], 5 is the start position and 0+1 the end position.

In slicing the start position must be lower than the end position. When this is not the case the result is an empty string. Thus f[0:5:-1] returns an empty string.