Policy on `is_foo` predicates

[Matthew Rocklin]

When making a predicate on a sympy class should we always use is_foo as in is_Real or should we use isfoo as in isinstance?  What is the standard style for denoting that something is a predicate?

This came up in https://github.com/sympy/sympy/pull/7346 for Set.issubset / Set.is_subset.

[Sergey B Kirpichev]

On Mon, Apr 28, 2014 at 06:39:09AM -0700, Matthew Rocklin wrote:
> When making a predicate on a sympy class should we always use is_foo as in
> is_Real or should we use isfoo as in isinstance?  What is the standard
> style for denoting that something is a predicate?

Well, here is an argument from statistics:
$ grep --exclude-dir='mpmath' -R 'def is_[a-z]' sympy/|wc -l
259

vs

$ grep --exclude-dir='mpmath' -R 'def is[a-z]' sympy/
sympy/core/evalf.py:def iszero(mpf, scaled=False):
sympy/utilities/lambdify.py: def isiter(l):
sympy/ntheory/primetest.py:def isprime(n):
sympy/polys/numberfields.py:def isolate(alg, eps=None, fast=False):

It seems, there is no isfoo *methods* at all.

[Aaron Meurer]

Yes, it seems the sympy way is to use the underscore. Also note that
almost all is_ attributes in sympy are either assumptions or class
classifications. The exceptions I know of are is_polynomial and
is_rationalfunction.

Aaron Meurer

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[Joachim Durchholz]

Am 28.04.2014 15:39, schrieb Matthew Rocklin:
> When making a predicate on a sympy class should we always use is_foo as in
> is_Real or should we use isfoo as in isinstance? What is the standard
> style for denoting that something is a predicate?

Here's a quick scan in my SymPy installation.
It's old-ish (six months) so current numbers may vary.

is with underscore:
$ cat `find -name "*.py"`|grep -c "def is[_]"
264

is without underscore:
$ cat `find -name "*.py"`|grep "def is[^_]"
22

So I guess the standard is is_ with an underscore.