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I believe there is a regression here.
numpy fails to build with Python 3.9.0a3.
________________________ TestMatmul.test_matmul_object _________________________
self = <numpy.core.tests.test_multiarray.TestMatmul object at 0x7ff9bc008ca0>
 def test_matmul_object(self):
 import fractions
 
 f = np.vectorize(fractions.Fraction)
 def random_ints():
 return np.random.randint(1, 1000, size=(10, 3, 3))
> M1 = f(random_ints(), random_ints())
f = <numpy.vectorize object at 0x7ff9bc6751f0>
fractions = <module 'fractions' from '/usr/lib64/python3.9/fractions.py'>
random_ints = <function TestMatmul.test_matmul_object.<locals>.random_ints at 0x7ff9bcc2d700>
self = <numpy.core.tests.test_multiarray.TestMatmul object at 0x7ff9bc008ca0>
numpy/core/tests/test_multiarray.py:6259: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
numpy/lib/function_base.py:2091: in __call__
 return self._vectorize_call(func=func, args=vargs)
numpy/lib/function_base.py:2161: in _vectorize_call
 ufunc, otypes = self._get_ufunc_and_otypes(func=func, args=args)
numpy/lib/function_base.py:2121: in _get_ufunc_and_otypes
 outputs = func(*inputs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
cls = <class 'fractions.Fraction'>, numerator = 483, denominator = 809
 def __new__(cls, numerator=0, denominator=None, *, _normalize=True):
 """Constructs a Rational.
 
 Takes a string like '3/2' or '1.5', another Rational instance, a
 numerator/denominator pair, or a float.
 
 Examples
 --------
 
 >>> Fraction(10, -8)
 Fraction(-5, 4)
 >>> Fraction(Fraction(1, 7), 5)
 Fraction(1, 35)
 >>> Fraction(Fraction(1, 7), Fraction(2, 3))
 Fraction(3, 14)
 >>> Fraction('314')
 Fraction(314, 1)
 >>> Fraction('-35/4')
 Fraction(-35, 4)
 >>> Fraction('3.1415') # conversion from numeric string
 Fraction(6283, 2000)
 >>> Fraction('-47e-2') # string may include a decimal exponent
 Fraction(-47, 100)
 >>> Fraction(1.47) # direct construction from float (exact conversion)
 Fraction(6620291452234629, 4503599627370496)
 >>> Fraction(2.25)
 Fraction(9, 4)
 >>> Fraction(Decimal('1.47'))
 Fraction(147, 100)
 
 """
 self = super(Fraction, cls).__new__(cls)
 
 if denominator is None:
 if type(numerator) is int:
 self._numerator = numerator
 self._denominator = 1
 return self
 
 elif isinstance(numerator, numbers.Rational):
 self._numerator = numerator.numerator
 self._denominator = numerator.denominator
 return self
 
 elif isinstance(numerator, (float, Decimal)):
 # Exact conversion
 self._numerator, self._denominator = numerator.as_integer_ratio()
 return self
 
 elif isinstance(numerator, str):
 # Handle construction from strings.
 m = _RATIONAL_FORMAT.match(numerator)
 if m is None:
 raise ValueError('Invalid literal for Fraction: %r' %
 numerator)
 numerator = int(m.group('num') or '0')
 denom = m.group('denom')
 if denom:
 denominator = int(denom)
 else:
 denominator = 1
 decimal = m.group('decimal')
 if decimal:
 scale = 10**len(decimal)
 numerator = numerator * scale + int(decimal)
 denominator *= scale
 exp = m.group('exp')
 if exp:
 exp = int(exp)
 if exp >= 0:
 numerator *= 10**exp
 else:
 denominator *= 10**-exp
 if m.group('sign') == '-':
 numerator = -numerator
 
 else:
 raise TypeError("argument should be a string "
 "or a Rational instance")
 
 elif type(numerator) is int is type(denominator):
 pass # *very* normal case
 
 elif (isinstance(numerator, numbers.Rational) and
 isinstance(denominator, numbers.Rational)):
 numerator, denominator = (
 numerator.numerator * denominator.denominator,
 denominator.numerator * numerator.denominator
 )
 else:
 raise TypeError("both arguments should be "
 "Rational instances")
 
 if denominator == 0:
 raise ZeroDivisionError('Fraction(%s, 0)' % numerator)
 if _normalize:
 if type(numerator) is int is type(denominator):
 # *very* normal case
 g = math.gcd(numerator, denominator)
 if denominator < 0:
 g = -g
 else:
> g = _gcd(numerator, denominator)
E NameError: name '_gcd' is not defined
__class__ = <class 'fractions.Fraction'>
_normalize = True
cls = <class 'fractions.Fraction'>
denominator = 809
numerator = 483
self = <[AttributeError("_numerator") raised in repr()] Fraction object at 0x7ff9bc6753a0>
/usr/lib64/python3.9/fractions.py:164: NameError
This removed _gcd, but it is still called in:
https://github.com/python/cpython/blob/58a4054760bffbb20aff90290dd0f3554f7bea42/Lib/fractions.py#L164