bpo-28626: rearrange discussion of output formatting to encourage f-strings (#6036)

The 'output formatting' section of the tutorial talks a lot about manual formatting with things like .rjust() and .zfill(), with only a passing reference to 3.6's new f-strings.

This doesn't drop all of the old material, but it does rearrange the topics into a more modern order: f-strings first, discussing formatting specifiers a bit; then calling .format(); finally manual formatting with .ljust().

Author: akuchling

Doc/tutorial/inputoutput.rst

@@ -20,20 +20,39 @@ of file objects; the standard output file can be referenced as ``sys.stdout``.
 See the Library Reference for more information on this.)
 
 Often you'll want more control over the formatting of your output than simply
-printing space-separated values.  There are two ways to format your output; the
-first way is to do all the string handling yourself; using string slicing and
-concatenation operations you can create any layout you can imagine.  The
-string type has some methods that perform useful operations for padding
-strings to a given column width; these will be discussed shortly.  The second
-way is to use :ref:`formatted string literals <f-strings>`, or the
-:meth:`str.format` method.
+printing space-separated values. There are several ways to format output.
 
-The :mod:`string` module contains a :class:`~string.Template` class which offers
-yet another way to substitute values into strings.
+* To use :ref:`formatted string literals <tut-f-strings>`, begin a string
+  with ``f`` or ``F`` before the opening quotation mark or triple quotation mark.
+  Inside this string, you can write a Python expression between ``{`` and ``}``
+  characters that can refer to variables or literal values.
 
-One question remains, of course: how do you convert values to strings? Luckily,
-Python has ways to convert any value to a string: pass it to the :func:`repr`
-or :func:`str` functions.
+  ::
+
+     >>> year = 2016 ; event = 'Referendum'
+     >>> f'Results of the {year} {event}'
+     'Results of the 2016 Referendum'
+
+* The :meth:`str.format` method of strings requires more manual
+  effort.  You'll still use ``{`` and ``}`` to mark where a variable
+  will be substituted and can provide detailed formatting directives,
+  but you'll also need to provide the information to be formatted.
+
+  ::
+
+     >>> yes_votes = 42_572_654 ; no_votes = 43_132_495
+     >>> percentage = (yes_votes/(yes_votes+no_votes)
+     >>> '{:-9} YES votes  {:2.2%}'.format(yes_votes, percentage))
+     ' 42572654 YES votes  49.67%'
+
+* Finally, you can do all the string handling yourself by using string slicing and
+  concatenation operations to create any layout you can imagine.  The
+  string type has some methods that perform useful operations for padding
+  strings to a given column width.
+
+When you don't need fancy output but just want a quick display of some
+variables for debugging purposes, you can convert any value to a string with
+the :func:`repr` or :func:`str` functions.
 
 The :func:`str` function is meant to return representations of values which are
 fairly human-readable, while :func:`repr` is meant to generate representations
@@ -67,60 +86,57 @@ Some examples::
    ... repr((x, y, ('spam', 'eggs')))
    "(32.5, 40000, ('spam', 'eggs'))"
 
-Here are two ways to write a table of squares and cubes::
+The :mod:`string` module contains a :class:`~string.Template` class that offers
+yet another way to substitute values into strings, using placeholders like
+``$x`` and replacing them with values from a dictionary, but offers much less
+control of the formatting.
 
-   >>> for x in range(1, 11):
-   ...     print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
-   ...     # Note use of 'end' on previous line
-   ...     print(repr(x*x*x).rjust(4))
-   ...
-    1   1    1
-    2   4    8
-    3   9   27
-    4  16   64
-    5  25  125
-    6  36  216
-    7  49  343
-    8  64  512
-    9  81  729
-   10 100 1000
 
-   >>> for x in range(1, 11):
-   ...     print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))
+.. _tut-f-strings:
+
+Formatted String Literals
+-------------------------
+
+:ref:`Formatted string literals <f-strings>` (also called f-strings for
+short) let you include the value of Python expressions inside a string by
+prefixing the string with ``f`` or ``F`` and writing expressions as
+``{expression}``.
+
+An optional format specifier can follow the expression. This allows greater
+control over how the value is formatted. The following example rounds pi to
+three places after the decimal::
+
+   >>> import math
+   >>> print(f'The value of pi is approximately {math.pi:.3f}.')
+
+Passing an integer after the ``':'`` will cause that field to be a minimum
+number of characters wide.  This is useful for making columns line up. ::
+
+   >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
+   >>> for name, phone in table.items():
+   ...     print(f'{name:10} ==> {phone:10d}')
    ...
-    1   1    1
-    2   4    8
-    3   9   27
-    4  16   64
-    5  25  125
-    6  36  216
-    7  49  343
-    8  64  512
-    9  81  729
-   10 100 1000
+   Sjoerd     ==>       4127
+   Jack       ==>       4098
+   Dcab       ==>       7678
 
-(Note that in the first example, one space between each column was added by the
-way :func:`print` works: by default it adds spaces between its arguments.)
+Other modifiers can be used to convert the value before it is formatted.
+``'!a'`` applies :func:`ascii`, ``'!s'`` applies :func:`str`, and ``'!r'``
+applies :func:`repr`::
 
-This example demonstrates the :meth:`str.rjust` method of string
-objects, which right-justifies a string in a field of a given width by padding
-it with spaces on the left.  There are similar methods :meth:`str.ljust` and
-:meth:`str.center`.  These methods do not write anything, they just return a
-new string.  If the input string is too long, they don't truncate it, but
-return it unchanged; this will mess up your column lay-out but that's usually
-better than the alternative, which would be lying about a value.  (If you
-really want truncation you can always add a slice operation, as in
-``x.ljust(n)[:n]``.)
+   >>> animals = 'eels'
+   >>> print(f'My hovercraft is full of {animals}.')
+   My hovercraft is full of eels.
+   >>> print('My hovercraft is full of {animals !r}.')
+   My hovercraft is full of 'eels'.
 
-There is another method, :meth:`str.zfill`, which pads a numeric string on the
-left with zeros.  It understands about plus and minus signs::
+For a reference on these format specifications, see
+the reference guide for the :ref:`formatspec`.
 
-   >>> '12'.zfill(5)
-   '00012'
-   >>> '-3.14'.zfill(7)
-   '-003.14'
-   >>> '3.14159265359'.zfill(5)
-   '3.14159265359'
+.. _tut-string-format:
+
+The String format() Method
+--------------------------
 
 Basic usage of the :meth:`str.format` method looks like this::
 
@@ -150,34 +166,6 @@ Positional and keyword arguments can be arbitrarily combined::
                                                           other='Georg'))
    The story of Bill, Manfred, and Georg.
 
-``'!a'`` (apply :func:`ascii`), ``'!s'`` (apply :func:`str`) and ``'!r'``
-(apply :func:`repr`) can be used to convert the value before it is formatted::
-
-   >>> contents = 'eels'
-   >>> print('My hovercraft is full of {}.'.format(contents))
-   My hovercraft is full of eels.
-   >>> print('My hovercraft is full of {!r}.'.format(contents))
-   My hovercraft is full of 'eels'.
-
-An optional ``':'`` and format specifier can follow the field name. This allows
-greater control over how the value is formatted.  The following example
-rounds Pi to three places after the decimal.
-
-   >>> import math
-   >>> print('The value of PI is approximately {0:.3f}.'.format(math.pi))
-   The value of PI is approximately 3.142.
-
-Passing an integer after the ``':'`` will cause that field to be a minimum
-number of characters wide.  This is useful for making tables pretty. ::
-
-   >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
-   >>> for name, phone in table.items():
-   ...     print('{0:10} ==> {1:10d}'.format(name, phone))
-   ...
-   Jack       ==>       4098
-   Dcab       ==>       7678
-   Sjoerd     ==>       4127
-
 If you have a really long format string that you don't want to split up, it
 would be nice if you could reference the variables to be formatted by name
 instead of by position.  This can be done by simply passing the dict and using
@@ -198,10 +186,71 @@ notation. ::
 This is particularly useful in combination with the built-in function
 :func:`vars`, which returns a dictionary containing all local variables.
 
+As an example, the following lines produce a tidily-aligned
+set of columns giving integers and their squares and cubes::
+
+   >>> for x in range(1, 11):
+   ...     print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))
+   ...
+    1   1    1
+    2   4    8
+    3   9   27
+    4  16   64
+    5  25  125
+    6  36  216
+    7  49  343
+    8  64  512
+    9  81  729
+   10 100 1000
+
 For a complete overview of string formatting with :meth:`str.format`, see
 :ref:`formatstrings`.
 
 
+Manual String Formatting
+------------------------
+
+Here's the same table of squares and cubes, formatted manually::
+
+   >>> for x in range(1, 11):
+   ...     print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
+   ...     # Note use of 'end' on previous line
+   ...     print(repr(x*x*x).rjust(4))
+   ...
+    1   1    1
+    2   4    8
+    3   9   27
+    4  16   64
+    5  25  125
+    6  36  216
+    7  49  343
+    8  64  512
+    9  81  729
+   10 100 1000
+
+(Note that the one space between each column was added by the
+way :func:`print` works: it always adds spaces between its arguments.)
+
+The :meth:`str.rjust` method of string objects right-justifies a string in a
+field of a given width by padding it with spaces on the left. There are
+similar methods :meth:`str.ljust` and :meth:`str.center`. These methods do
+not write anything, they just return a new string. If the input string is too
+long, they don't truncate it, but return it unchanged; this will mess up your
+column lay-out but that's usually better than the alternative, which would be
+lying about a value. (If you really want truncation you can always add a
+slice operation, as in ``x.ljust(n)[:n]``.)
+
+There is another method, :meth:`str.zfill`, which pads a numeric string on the
+left with zeros.  It understands about plus and minus signs::
+
+   >>> '12'.zfill(5)
+   '00012'
+   >>> '-3.14'.zfill(7)
+   '-003.14'
+   >>> '3.14159265359'.zfill(5)
+   '3.14159265359'
+
+
 Old string formatting
 ---------------------
 
@@ -211,8 +260,8 @@ to the right argument, and returns the string resulting from this formatting
 operation. For example::
 
    >>> import math
-   >>> print('The value of PI is approximately %5.3f.' % math.pi)
-   The value of PI is approximately 3.142.
+   >>> print('The value of pi is approximately %5.3f.' % math.pi)
+   The value of pi is approximately 3.142.
 
 More information can be found in the :ref:`old-string-formatting` section.