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gh-85566: State interpreter var "_" in appendix of tutorial #21654

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gh-85566: State interpreter var "_" in appendix of tutorial #21654

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Create 2020-07-27-23-02-27.bpo-41394.BEos8A.rst
wyz23x2 Jul 28, 2020
815fad9
Update appendix.rst
wyz23x2 Jul 28, 2020
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Added space around "+"
wyz23x2 Jul 28, 2020
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Remove trailing whitespace on line 39
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Update 2020-07-27-23-02-27.bpo-41394.BEos8A.rst
wyz23x2 Jul 28, 2020
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Update 2020-07-27-23-02-27.bpo-41394.BEos8A.rst
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Merge branch 'main' into master
skirpichev Apr 19, 2025
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26 changes: 26 additions & 0 deletions Doc/tutorial/appendix.rst
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Expand Up @@ -31,6 +31,32 @@ Typing an interrupt while a command is executing raises the
:exc:`KeyboardInterrupt` exception, which may be handled by a :keyword:`try`
statement.

.. _tut-var:
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What about something less generic / cryptic:

Suggested change
.. _tut-var:
.. _tut-underscore-variable:


Special ``_`` variable
-------------------------

There is a special variable ``_`` in interactive mode, which stores the result of the last
evaluated expression. If the last expression evaluated to ``None``, then the value of ``_``
is not updated.

For example:

>>> _ # Not defined yet
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
_
NameError: name '_' is not defined
Comment on lines +66 to +71
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Suggested change
>>> _ # Not defined yet
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
_
NameError: name '_' is not defined
.. doctest::
:options: +SKIP
>>> _ # Not defined yet
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
_
NameError: name '_' is not defined

I doubt this example is fixable. Either you could remove one or exclude entire block from testing, as suggested.

>>> 1 + 2
3
>>> _
3
>>> print("1+2=3")
1+2=3
>>> _ # Doesn't store None returned by print()
3
>>> _+4
7

.. _tut-scripts:

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Documentation/2020-07-27-23-02-27.bpo-41394.BEos8A.rst
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@@ -0,0 +1,2 @@
Added "Special ``_`` Variable`` section to the 16. Appendix part of Python tutorial.
It includes the define of ``_`` and an example.