You should really just write your own function -- Python can't include every validation function you can think of. It already provides an extensible and well tested float conversion which throws an exception on bad input (the 'float' constructor)

You want it to not throw an exception, but rather return a status indicating success. This is called Look Before You Leap (LBYL) and is considered un-Pythonic (i.e. is not standard practice while writing Python). 

If you absolutely must have it in this format (which you shouldn't), you should just write a function that accepts a string, and has a try/except block and then returns the status. Again, don't do this, it's not good practice:

def parsefloat(s):
    try:
        return float(s)
    except:
        return none # or whatever you want to indicate failure

I think the confusion of the 'isdigit()' method (and thus your original inquiry into a 'isfloat()' meyhod) stems from a misunderstanding; 'isdigit()' tests whether characters are in a Unicode character set or not -- it doesn't actually convert to a number.

Parsing floats is a more complicated and different problem, which doesn't belong in the 'str' class, and already does exist in the 'float' class.

Although you say that try/except blocks are ugly/unnecessary/etc, try programming with LBYL idioms as you've suggested. I think you'll find they're not only more verbose, less necessary, and uglier, but they also make exception shadowing much easier (and thus, proper exception handling harder to do).

On Sun, Dec 27, 2020, 5:58 PM Joao S. O. Bueno <jsbueno@python.org.br> wrote:


On Sun, 27 Dec 2020 at 19:31, Chris Angelico <rosuav@gmail.com> wrote:
On Mon, Dec 28, 2020 at 9:22 AM Joao S. O. Bueno <jsbueno@python.org.br> wrote:
>
> I agree - the three builtin methods are almost the same (not sure if
> there is any difference at all),

Yes - they all check if the string matches a particular set of characters.

> while there is no trivial way to check for a valid
> float, or otherwise a chosen  representation of a decimal number without resorting
> to a try-except statement, or complicated verification schemes that have
> to deal with a lot of corner cases.

If you want to know whether the float() call will throw, the best way
to check is to call float() and see if it throws.

> For validity one has to check if there are only digits - and decimal points -
> and the "-" unary sign. What if there is more of a "." in the string?
> what if the  "-" is not the first character?
> And besides validity checking, there are also validity choices:
>  What if an unary "+" is present? Whitespace ok? "_" as digit separator ok?
> scientific exponential notation accepted?  What about Inf and Nan literals?
> What about taking into account the locale setting?
>
> But maybe, instead of yet another str method, a "parsefloat" function
> that could get arguments and sensitive defaults for some choices -
> it could live in "math" or "numbers".

How about a built-in called "float", which either returns the
floating-point value represented by the string, or signals an invalid
string with an exception?

> I think it would be preferable than, say, adding a lot of options
> to the `float` constructor. These are the options I can think
> of the top of my mind:

YAGNI. If you want to allow specific subsets of valid options, it's
not that hard to do your own validation.

> The return value would consitss of a boolean, where True would indicate success, and then the number.
> Or it could return a single float, returning a NaN in case of parsing error.

Or it could raise in the case of parsing error. That's the Pythonic way.

Sorry, I thought my message conveyed that I know "float" exists, and
try/except is the current usable pattern (it is in the original posting anyway)

I tried to make clear this should be in addition to that - 
But yes, I failed to mention in my message that I think such a function
would mostly  benefit beginners learning around with "input" and "print" -
it is painful to suddenly have to tour the students  on several other concepts just 
to get a correct user-inputed number. (OTOH, yes, for code on this level,
one normally won't be concerned if the program user will be typing "1.02e2" on
the `input` prompt).

The point is exactly that parsing a number correctly, and moreover respecting
these options, is subject to error and the stdlib could benefit from 
a construct that would not require a try/except block for everything. 
(As you can see, I contemplate that raising may be a desired option for
a flexible function, and there is an option for that in my example signature) .  





ChrisA
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