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bpo-41394: State special _ in appendix of tutorial #21651

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a8fc1af
Update appendix.rst
wyz23x2 Jul 28, 2020
d86b9e0
Fixed hyper to match "tut"
wyz23x2 Jul 28, 2020
7a56efd
Update appendix.rst
wyz23x2 Jul 28, 2020
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Create 2020-07-27-23-05-02.bpo-41394.BEos8A.rst
wyz23x2 Jul 28, 2020
8b4d79e
bpo-41394: Document '_' in interpreter tutorial
wyz23x2 Jul 28, 2020
81dab86
Update 2020-07-27-23-05-02.bpo-41394.BEos8A.rst
wyz23x2 Jul 28, 2020
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bpo-41394: State special _ in appendix of tutorial
wyz23x2 Jul 28, 2020
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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:

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

There is a special variable ``_`` in interactive mode, stored in the :mod:`builtins`
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Is it really true that _ is stored in builtins? I played around with it a while, and couldn't verify that. And even if it's true, is it important? I don't think so. Do you have another reference to _ being in builtins?

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This was stated in bpo-41394 too.
https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers

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Interesting. I still think it's unimportant for the tutorial and should be removed.

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OK. Working on it.

module. It stores the result of the last evaluated expression (*except* ``None``
— it remains unchanged).
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@ericvsmith ericvsmith Jul 28, 2020 •
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I think this would read better as: "It stores the result of the last evaluated expression. If the last expression evaluated to None, then the value of _ is not updated."

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Agree.


For example:

>>> _ # Not defined yet
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
_
NameError: name '_' is not defined
>>> 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/2020-07-27-23-05-02.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 definition of ``_`` and an example.