ENH: Add support for symbol to polynomial package (#16154)

Adds a symbol attribute to the polynomials from the np.polynomial package to allow the user to control/modify the symbol used to represent the independent variable for a polynomial expression. This attribute corresponds to the variable attribute of the poly1d class from the old np.lib.polynomial module.

Marked as draft for now as it depends on #15666 - all _str* and _repr* methods of ABCPolyBase and derived classes would need to be modified (and tested) to support this change.

Co-authored-by: Warren Weckesser <warren.weckesser@gmail.com>

2 files changed, 289 insertions, 6 deletions

diff --git a/numpy/polynomial/tests/test_printing.py b/numpy/polynomial/tests/test_printing.py
index 4e9902a69..0c4316223 100644
--- a/numpy/polynomial/tests/test_printing.py
+++ b/numpy/polynomial/tests/test_printing.py
@@ -309,35 +309,66 @@ class TestFormat:
             format(p, '.2f')
 
 
+@pytest.mark.parametrize(('poly', 'tgt'), (
+    (poly.Polynomial, '1.0 + 2.0·z¹ + 3.0·z²'),
+    (poly.Chebyshev, '1.0 + 2.0·T₁(z) + 3.0·T₂(z)'),
+    (poly.Hermite, '1.0 + 2.0·H₁(z) + 3.0·H₂(z)'),
+    (poly.HermiteE, '1.0 + 2.0·He₁(z) + 3.0·He₂(z)'),
+    (poly.Laguerre, '1.0 + 2.0·L₁(z) + 3.0·L₂(z)'),
+    (poly.Legendre, '1.0 + 2.0·P₁(z) + 3.0·P₂(z)'),
+))
+def test_symbol(poly, tgt):
+    p = poly([1, 2, 3], symbol='z')
+    assert_equal(f"{p:unicode}", tgt)
+
+
 class TestRepr:
     def test_polynomial_str(self):
         res = repr(poly.Polynomial([0, 1]))
-        tgt = 'Polynomial([0., 1.], domain=[-1,  1], window=[-1,  1])'
+        tgt = (
+            "Polynomial([0., 1.], domain=[-1,  1], window=[-1,  1], "
+            "symbol='x')"
+        )
         assert_equal(res, tgt)
 
     def test_chebyshev_str(self):
         res = repr(poly.Chebyshev([0, 1]))
-        tgt = 'Chebyshev([0., 1.], domain=[-1,  1], window=[-1,  1])'
+        tgt = (
+            "Chebyshev([0., 1.], domain=[-1,  1], window=[-1,  1], "
+            "symbol='x')"
+        )
         assert_equal(res, tgt)
 
     def test_legendre_repr(self):
         res = repr(poly.Legendre([0, 1]))
-        tgt = 'Legendre([0., 1.], domain=[-1,  1], window=[-1,  1])'
+        tgt = (
+            "Legendre([0., 1.], domain=[-1,  1], window=[-1,  1], "
+            "symbol='x')"
+        )
         assert_equal(res, tgt)
 
     def test_hermite_repr(self):
         res = repr(poly.Hermite([0, 1]))
-        tgt = 'Hermite([0., 1.], domain=[-1,  1], window=[-1,  1])'
+        tgt = (
+            "Hermite([0., 1.], domain=[-1,  1], window=[-1,  1], "
+            "symbol='x')"
+        )
         assert_equal(res, tgt)
 
     def test_hermiteE_repr(self):
         res = repr(poly.HermiteE([0, 1]))
-        tgt = 'HermiteE([0., 1.], domain=[-1,  1], window=[-1,  1])'
+        tgt = (
+            "HermiteE([0., 1.], domain=[-1,  1], window=[-1,  1], "
+            "symbol='x')"
+        )
         assert_equal(res, tgt)
 
     def test_laguerre_repr(self):
         res = repr(poly.Laguerre([0, 1]))
-        tgt = 'Laguerre([0., 1.], domain=[0, 1], window=[0, 1])'
+        tgt = (
+            "Laguerre([0., 1.], domain=[0, 1], window=[0, 1], "
+            "symbol='x')"
+        )
         assert_equal(res, tgt)
 
 
@@ -388,3 +419,39 @@ class TestLatexRepr:
         p = poly.HermiteE([1, 2, 3])
         assert_equal(self.as_latex(p),
             r'$x \mapsto 1.0\,{He}_{0}(x) + 2.0\,{He}_{1}(x) + 3.0\,{He}_{2}(x)$')
+
+    def test_symbol_basic(self):
+        # default input
+        p = poly.Polynomial([1, 2, 3], symbol='z')
+        assert_equal(self.as_latex(p),
+            r'$z \mapsto 1.0 + 2.0\,z + 3.0\,z^{2}$')
+
+        # translated input
+        p = poly.Polynomial([1, 2, 3], domain=[-2, 0], symbol='z')
+        assert_equal(
+            self.as_latex(p),
+            (
+                r'$z \mapsto 1.0 + 2.0\,\left(1.0 + z\right) + 3.0\,'
+                r'\left(1.0 + z\right)^{2}$'
+            ),
+        )
+
+        # scaled input
+        p = poly.Polynomial([1, 2, 3], domain=[-0.5, 0.5], symbol='z')
+        assert_equal(
+            self.as_latex(p),
+            (
+                r'$z \mapsto 1.0 + 2.0\,\left(2.0z\right) + 3.0\,'
+                r'\left(2.0z\right)^{2}$'
+            ),
+        )
+
+        # affine input
+        p = poly.Polynomial([1, 2, 3], domain=[-1, 0], symbol='z')
+        assert_equal(
+            self.as_latex(p),
+            (
+                r'$z \mapsto 1.0 + 2.0\,\left(1.0 + 2.0z\right) + 3.0\,'
+                r'\left(1.0 + 2.0z\right)^{2}$'
+            ),
+        )
diff --git a/numpy/polynomial/tests/test_symbol.py b/numpy/polynomial/tests/test_symbol.py
new file mode 100644
index 000000000..4ea6035ef
--- /dev/null
+++ b/numpy/polynomial/tests/test_symbol.py
@@ -0,0 +1,216 @@
+"""
+Tests related to the ``symbol`` attribute of the ABCPolyBase class.
+"""
+
+import pytest
+import numpy.polynomial as poly
+from numpy.core import array
+from numpy.testing import assert_equal, assert_raises, assert_
+
+
+class TestInit:
+    """
+    Test polynomial creation with symbol kwarg.
+    """
+    c = [1, 2, 3]
+
+    def test_default_symbol(self):
+        p = poly.Polynomial(self.c)
+        assert_equal(p.symbol, 'x')
+
+    @pytest.mark.parametrize(('bad_input', 'exception'), (
+        ('', ValueError),
+        ('3', ValueError),
+        (None, TypeError),
+        (1, TypeError),
+    ))
+    def test_symbol_bad_input(self, bad_input, exception):
+        with pytest.raises(exception):
+            p = poly.Polynomial(self.c, symbol=bad_input)
+
+    @pytest.mark.parametrize('symbol', (
+        'x',
+        'x_1',
+        'A',
+        'xyz',
+        'β',
+    ))
+    def test_valid_symbols(self, symbol):
+        """
+        Values for symbol that should pass input validation.
+        """
+        p = poly.Polynomial(self.c, symbol=symbol)
+        assert_equal(p.symbol, symbol)
+
+    def test_property(self):
+        """
+        'symbol' attribute is read only.
+        """
+        p = poly.Polynomial(self.c, symbol='x')
+        with pytest.raises(AttributeError):
+            p.symbol = 'z'
+
+    def test_change_symbol(self):
+        p = poly.Polynomial(self.c, symbol='y')
+        # Create new polynomial from p with different symbol
+        pt = poly.Polynomial(p.coef, symbol='t')
+        assert_equal(pt.symbol, 't')
+
+
+class TestUnaryOperators:
+    p = poly.Polynomial([1, 2, 3], symbol='z')
+
+    def test_neg(self):
+        n = -self.p
+        assert_equal(n.symbol, 'z')
+
+    def test_scalarmul(self):
+        out = self.p * 10
+        assert_equal(out.symbol, 'z')
+
+    def test_rscalarmul(self):
+        out = 10 * self.p
+        assert_equal(out.symbol, 'z')
+
+    def test_pow(self):
+        out = self.p ** 3
+        assert_equal(out.symbol, 'z')
+
+
+@pytest.mark.parametrize(
+    'rhs',
+    (
+        poly.Polynomial([4, 5, 6], symbol='z'),
+        array([4, 5, 6]),
+    ),
+)
+class TestBinaryOperatorsSameSymbol:
+    """
+    Ensure symbol is preserved for numeric operations on polynomials with
+    the same symbol
+    """
+    p = poly.Polynomial([1, 2, 3], symbol='z')
+
+    def test_add(self, rhs):
+        out = self.p + rhs
+        assert_equal(out.symbol, 'z')
+
+    def test_sub(self, rhs):
+        out = self.p - rhs
+        assert_equal(out.symbol, 'z')
+
+    def test_polymul(self, rhs):
+        out = self.p * rhs
+        assert_equal(out.symbol, 'z')
+
+    def test_divmod(self, rhs):
+        for out in divmod(self.p, rhs):
+            assert_equal(out.symbol, 'z')
+
+    def test_radd(self, rhs):
+        out = rhs + self.p
+        assert_equal(out.symbol, 'z')
+
+    def test_rsub(self, rhs):
+        out = rhs - self.p
+        assert_equal(out.symbol, 'z')
+
+    def test_rmul(self, rhs):
+        out = rhs * self.p
+        assert_equal(out.symbol, 'z')
+
+    def test_rdivmod(self, rhs):
+        for out in divmod(rhs, self.p):
+            assert_equal(out.symbol, 'z')
+
+
+class TestBinaryOperatorsDifferentSymbol:
+    p = poly.Polynomial([1, 2, 3], symbol='x')
+    other = poly.Polynomial([4, 5, 6], symbol='y')
+    ops = (p.__add__, p.__sub__, p.__mul__, p.__floordiv__, p.__mod__)
+
+    @pytest.mark.parametrize('f', ops)
+    def test_binops_fails(self, f):
+        assert_raises(ValueError, f, self.other)
+
+
+class TestEquality:
+    p = poly.Polynomial([1, 2, 3], symbol='x')
+
+    def test_eq(self):
+        other = poly.Polynomial([1, 2, 3], symbol='x')
+        assert_(self.p == other)
+
+    def test_neq(self):
+        other = poly.Polynomial([1, 2, 3], symbol='y')
+        assert_(not self.p == other)
+
+
+class TestExtraMethods:
+    """
+    Test other methods for manipulating/creating polynomial objects.
+    """
+    p = poly.Polynomial([1, 2, 3, 0], symbol='z')
+
+    def test_copy(self):
+        other = self.p.copy()
+        assert_equal(other.symbol, 'z')
+
+    def test_trim(self):
+        other = self.p.trim()
+        assert_equal(other.symbol, 'z')
+
+    def test_truncate(self):
+        other = self.p.truncate(2)
+        assert_equal(other.symbol, 'z')
+
+    @pytest.mark.parametrize('kwarg', (
+        {'domain': [-10, 10]},
+        {'window': [-10, 10]},
+        {'kind': poly.Chebyshev},
+    ))
+    def test_convert(self, kwarg):
+        other = self.p.convert(**kwarg)
+        assert_equal(other.symbol, 'z')
+
+    def test_integ(self):
+        other = self.p.integ()
+        assert_equal(other.symbol, 'z')
+
+    def test_deriv(self):
+        other = self.p.deriv()
+        assert_equal(other.symbol, 'z')
+
+
+def test_composition():
+    p = poly.Polynomial([3, 2, 1], symbol="t")
+    q = poly.Polynomial([5, 1, 0, -1], symbol="λ_1")
+    r = p(q)
+    assert r.symbol == "λ_1"
+
+
+#
+# Class methods that result in new polynomial class instances
+#
+
+
+def test_fit():
+    x, y = (range(10),)*2
+    p = poly.Polynomial.fit(x, y, deg=1, symbol='z')
+    assert_equal(p.symbol, 'z')
+
+
+def test_froomroots():
+    roots = [-2, 2]
+    p = poly.Polynomial.fromroots(roots, symbol='z')
+    assert_equal(p.symbol, 'z')
+
+
+def test_identity():
+    p = poly.Polynomial.identity(domain=[-1, 1], window=[5, 20], symbol='z')
+    assert_equal(p.symbol, 'z')
+
+
+def test_basis():
+    p = poly.Polynomial.basis(3, symbol='z')
+    assert_equal(p.symbol, 'z')