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From: Christopher Reimer <christopher_reimer@icloud.com>
Newsgroups: comp.lang.python
Subject: Re: Creating a calculator
Date: Fri, 01 Jul 2016 05:25:58 -0700
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> On Jun 30, 2016, at 11:42 PM, Jussi Piitulainen <jussi.piitulainen@helsink=
i.fi> wrote:
>=20
> DFS writes:
>=20
>> Here's a related program that doesn't require you to tell it what type
>> of operation to perform.  Just enter 'num1 operator num2' and hit
>> Enter, and it will parse the entry and do the math.
>>=20
>> -----------------------------------------------
>> ui=3Draw_input('Enter calculation to perform: ')
>> n1=3Dfloat(ui.split(' ')[0])
>> op=3Dui.split(' ')[1]
>> n2=3Dfloat(ui.split(' ')[2])
>> if op=3D=3D'+':c=3Dn1+n2
>> if op=3D=3D'-':c=3Dn1-n2
>> if op=3D=3D'*':c=3Dn1*n2
>> if op=3D=3D'/':c=3Dn1/n2
>> print(ui+' =3D '+str(c))
>> -----------------------------------------------
>=20
> I use multiple assignment a lot, like this:
>=20
>    n1, op, n2 =3D ui.split()
>=20
> It's not only compact, it also crashes if there are more elements than
> expected, and I want it to crash when that happens. Or rather, I prefer
> a crash to silence when input is bad.
>=20
> For the calculator, it may be better to split on any whitespace and
> discard empty strings, which is what ui.split() does. Splitting on a
> single space seems unnecessarily strict in a calculator (whereas
> splitting on a single tab is what I very much do in my work - the data
> formats are such).
>=20
> I think multiple assignment is good even for a beginner. Perhaps do it a
> second time straight away:
>=20
>    n1, op, n2 =3D ui.split()
>    n1, n2 =3D float(n1), float(n2)
>=20
> But it's only with the split where it really pays.
>=20
>    n1, op, n2 =3D ui.split()
>    n1 =3D float(n1)
>    n2 =3D float(n2)
>=20
> The latter might be even preferable. Hm.
>=20
>    n1, n2 =3D map(float, (n1, n2))
>=20
> :)

For my BASIC interpreter, each line of BASIC is broken this way into tokens.=


line_number, keyword, *expression =3D line.split(' ', 2)

For a line like 10 PRINT "HELLO, WORLD!", this works as expected.

For a line like 20 END, which doesn't have a third element for expression, a=
n empty list is assigned.

By using * to unpack the split line, my program no longer crashes and no try=
/except block is needed to work around the crash. A later line of code will t=
est the expression, ignore if empty or run regex if full.=20

Chris R.=