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Minor update to the PEP #17

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merged 3 commits into from
Oct 16, 2024
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Minor update to the PEP #17

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22ac80e
Minor update to the PEP
lysnikolaou Oct 16, 2024
f5cf6d1
Explain eager evaluation of interpolated format specifiers in t-strings.
davepeck Oct 16, 2024
45ef598
Slightly fewer words.
davepeck Oct 16, 2024
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25 changes: 18 additions & 7 deletions peps/pep-0750.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -115,7 +115,7 @@ The ``Template`` Type
---------------------

Template strings evaluate to an instance of a new type, ``Template``, found
in the ``types`` module:
in the ``<TBC>`` module (proposed: ``types``):

.. code-block:: python

Expand All @@ -137,7 +137,7 @@ any interpolations in the literal:

The use of ``@dataclass`` in the definition of ``Template`` above (and
``Interpolation`` below) is meant to be suggestive; the exact implementation
in ``cpython`` may differ, but developers can expect that ``Template`` instances
in CPython may differ, but developers can expect that ``Template`` instances
can be constructed and utilized in the same way as a typical dataclass.

See `Interleaving of Template.args`_ below for more information on how the
Expand All @@ -154,7 +154,7 @@ Like ``Template``, it is a new concrete type found in the ``types`` module:

@dataclass(frozen=True)
class Interpolation:
value: Any
value: object
expr: str
conv: Literal["a", "r", "s"] | None = None
format_spec: str = ""
Expand Down Expand Up @@ -202,6 +202,17 @@ As with f-strings, this is an arbitrary string that defines how to present the v
template = t"Value: {value:.2f}"
assert template.args[1].format_spec == ".2f"

Format specifications in f-strings can themselves contain interpolations. This
is permitted in template strings as well; ``format_spec`` is set to the eagerly
evaluated result:

.. code-block:: python

value = 42
precision = 2
template = t"Value: {value:.{precision}f}"
assert template.args[1].format_spec == ".2f"

If no format specification is provided, ``format_spec`` defaults to an empty
string (``""``). This matches the ``format_spec`` parameter of Python's
`format() <https://docs.python.org/3/library/functions.html#format>`_ built-in.
Expand Down Expand Up @@ -273,8 +284,8 @@ a ``SyntaxError``:
Support for the Debug Specifier
-------------------------------

The debug specifier, ``=``, is supported in template strings but behaves
slightly differently than in f-strings. The specifier
The debug specifier, ``=``, is supported in template strings and behaves similarly
to how it behaves in f-strings. The specifier
was introduced in `gh-80998 <https://github.com/python/cpython/issues/80998>`_
outside of any PEP. The distinction in behavior is due to technical limitations
of the implementation.
Expand Down Expand Up @@ -1022,9 +1033,9 @@ source text:

.. code-block:: python

spec = ".2f"
value = 42
template = t"{value:{spec}}"
precision = 2
template = t"Value: {value:.{precision}f}"
assert template.args[1].format_spec == ".2f"

We do not anticipate that these limitations will be a significant issue in practice.
Expand Down