Why is it not possible to convert "1.7" to integer directly, without converting to float first?

Question

When I type int("1.7") Python returns error (specifically, ValueError). I know that I can convert it to integer by int(float("1.7")). I would like to know why the first method returns error.

Answer 1

From the documentation:

If x is not a number or if base is given, then x must be a string or Unicode object representing an integer literal in radix base ...

Obviously, "1.7" does not represent an integer literal in radix base.

If you want to know why the python dev's decided to limit themselves to integer literals in radix base, there are a possible infinite number of reasons and you'd have to ask Guido et. al to know for sure. One guess would be ease of implementation + efficiency. You might think it would be easily for them to implement it as:

1. Interpret number as a float
2. truncate to an integer

Unfortunately, that doesn't work in python as integers can have arbitrary precision and floats cannot. Special casing big numbers could lead to inefficiency for the common case¹.

Additionally, forcing you do to int(float(...)) has the additional benefit in clarity -- It makes it more obvious what the input string probably looks like which can help in debugging elsewhere. In fact, I might argue that even if int would accept strings like "1.7", it'd be better to write int(float("1.7")) anyway for the increased code clarity.

^{1Assuming some validation. Other languages skip this -- e.g. ruby will evaluate '1e6'.to_i and give you 1 since it stops parsing at the first non-integral character. Seems like that could lead to fun bugs to track down ...}