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bpo-40269: Add note on imaginary literal gotchas to reference documentation #19498

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c73a2fc
Add note on imaginary literals clarifying some surprising corner cases
mdickinson Apr 13, 2020
7bc4d87
Add second example of using complex constructor
mdickinson Apr 13, 2020
ff44502
Link properly to the complex constructor
mdickinson Apr 13, 2020
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28 changes: 28 additions & 0 deletions Doc/reference/lexical_analysis.rst
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Expand Up @@ -878,6 +878,34 @@ imaginary literals::

3.14j 10.j 10j .001j 1e100j 3.14e-10j 3.14_15_93j

.. note::

An imaginary literal cannot include a leading ``-`` sign. The expression
``-1j`` is interpreted as the unary minus operator applied to the literal
``1j``. The unary minus negates both the real and imaginary parts,
so ``-1j`` is a complex number with real part ``-0.0`` and imaginary
part ``-1.0``::

>>> -1j
(-0-1j)

Similarly, in the addition ``-0.0 + 1j``, the left addend ``-0.0`` is
promoted to a complex number with real part ``-0.0`` and imaginary part
``0.0``, while ``1j`` has real part ``0.0``. The sum of the real parts is
thus ``0.0``, not ``-0.0``::

>>> -0.0 + 1j
1j

To avoid these and similar difficulties related to signs of zeros, a
complex number can be created directly from its real and imaginary parts
using the :func:`complex` constructor::

>>> complex(0.0, -1.0)
-1j
>>> complex(-0.0, 1.0)
(-0+1j)


.. _operators:

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