gh-39426: Minor code improvement in `sage.doctest`
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes #12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes #12345". -->

Minor revision, adding a bit of typing info, resorting imports and
resolving some name collisions / overwrites.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [ ] The title is concise and informative.
- [ ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #39426
Reported by: Tobias Diez
Reviewer(s): Sébastien Labbé

Author: Release Manager

src/sage/doctest/control.py

@@ -32,28 +32,31 @@
 # ****************************************************************************
 
 import importlib
-import random
+import json
 import os
+import random
+import shlex
 import sys
 import time
-import json
-import shlex
 import types
+
+from cysignals.signals import AlarmInterrupt, init_cysignals
+
 import sage.misc.flatten
 import sage.misc.randstate as randstate
-from sage.structure.sage_object import SageObject
-from sage.env import DOT_SAGE, SAGE_LIB, SAGE_SRC, SAGE_VENV, SAGE_EXTCODE
+from sage.doctest.external import available_software
+from sage.doctest.forker import DocTestDispatcher
+from sage.doctest.parsing import (
+    optional_tag_regex,
+    parse_file_optional_tags,
+    unparse_optional_tags,
+)
+from sage.doctest.reporting import DocTestReporter
+from sage.doctest.sources import DictAsObject, FileDocTestSource, get_basename
+from sage.doctest.util import Timer, count_noun, dict_difference
+from sage.env import DOT_SAGE, SAGE_EXTCODE, SAGE_LIB, SAGE_SRC
 from sage.misc.temporary_file import tmp_dir
-from cysignals.signals import AlarmInterrupt, init_cysignals
-
-from .sources import FileDocTestSource, DictAsObject, get_basename
-from .forker import DocTestDispatcher
-from .reporting import DocTestReporter
-from .util import Timer, count_noun, dict_difference
-from .external import available_software
-from .parsing import parse_optional_tags, parse_file_optional_tags, unparse_optional_tags, \
-     nodoctest_regex, optionaltag_regex, optionalfiledirective_regex
-
+from sage.structure.sage_object import SageObject
 
 # Optional tags which are always automatically added
 
@@ -465,7 +468,7 @@ def __init__(self, options, args):
                 s = options.hide.lower()
                 options.hide = set(s.split(','))
                 for h in options.hide:
-                    if not optionaltag_regex.search(h):
+                    if not optional_tag_regex.search(h):
                         raise ValueError('invalid optional tag {!r}'.format(h))
             if 'all' in options.hide:
                 options.hide.discard('all')
@@ -508,10 +511,10 @@ def __init__(self, options, args):
                 # Check that all tags are valid
                 for o in options.optional:
                     if o.startswith('!'):
-                        if not optionaltag_regex.search(o[1:]):
+                        if not optional_tag_regex.search(o[1:]):
                             raise ValueError('invalid optional tag {!r}'.format(o))
                         options.disabled_optional.add(o[1:])
-                    elif not optionaltag_regex.search(o):
+                    elif not optional_tag_regex.search(o):
                         raise ValueError('invalid optional tag {!r}'.format(o))
 
                 options.optional |= auto_optional_tags
@@ -531,7 +534,7 @@ def __init__(self, options, args):
                 else:
                     # Check that all tags are valid
                     for o in options.probe:
-                        if not optionaltag_regex.search(o):
+                        if not optional_tag_regex.search(o):
                             raise ValueError('invalid optional tag {!r}'.format(o))
 
         self.options = options

src/sage/doctest/external.py

@@ -391,7 +391,7 @@ def external_features():
     yield Gurobi()
 
 
-def external_software() -> list[str]:
+def _external_software() -> list[str]:
     """
     Return the alphabetical list of external software supported by this module.
 
@@ -404,7 +404,7 @@ def external_software() -> list[str]:
     return sorted(f.name for f in external_features())
 
 
-external_software = external_software()
+external_software: list[str] = _external_software()
 
 
 class AvailableSoftware:

src/sage/doctest/forker.py

@@ -248,7 +248,7 @@ def init_sage(controller: DocTestController | None = None) -> None:
 
     try:
         import sympy
-    except ImportError:
+    except (ImportError, AttributeError):
         # Do not require sympy for running doctests (Issue #25106).
         pass
     else:

src/sage/doctest/parsing.py

@@ -39,36 +39,44 @@
 import re
 from collections import defaultdict
 from functools import reduce
+from re import Pattern
 from typing import Literal, Union, overload
 
+from sage.doctest.check_tolerance import (
+    ToleranceExceededError,
+    check_tolerance_complex_domain,
+    check_tolerance_real_domain,
+    float_regex,
+)
+from sage.doctest.external import available_software, external_software
+from sage.doctest.marked_output import MarkedOutput
+from sage.doctest.rif_tol import RIFtol, add_tolerance
 from sage.misc.cachefunc import cached_function
 from sage.repl.preparse import preparse, strip_string_literals
-from sage.doctest.rif_tol import RIFtol, add_tolerance
-from sage.doctest.marked_output import MarkedOutput
-from sage.doctest.check_tolerance import (
-    ToleranceExceededError, check_tolerance_real_domain,
-    check_tolerance_complex_domain, float_regex)
-
-from .external import available_software, external_software
-
 
 # This is the correct pattern to match ISO/IEC 6429 ANSI escape sequences:
-ansi_escape_sequence = re.compile(r"(\x1b[@-Z\\-~]|\x1b\[.*?[@-~]|\x9b.*?[@-~])")
+ansi_escape_sequence: Pattern[str] = re.compile(
+    r"(\x1b[@-Z\\-~]|\x1b\[.*?[@-~]|\x9b.*?[@-~])"
+)
 
-special_optional_regex = (
+special_optional_regex_raw = (
     "py2|long time|not implemented|not tested|optional|needs|known bug"
 )
-tag_with_explanation_regex = r"((?:!?\w|[.])*)\s*(?:\((?P<cmd_explanation>.*?)\))?"
-optional_regex = re.compile(
-    rf"[^ a-z]\s*(?P<cmd>{special_optional_regex})(?:\s|[:-])*(?P<tags>(?:(?:{tag_with_explanation_regex})\s*)*)",
+tag_with_explanation_regex_raw = r"((?:!?\w|[.])*)\s*(?:\((?P<cmd_explanation>.*?)\))?"
+optional_regex: Pattern[str] = re.compile(
+    rf"[^ a-z]\s*(?P<cmd>{special_optional_regex_raw})(?:\s|[:-])*(?P<tags>(?:(?:{tag_with_explanation_regex_raw})\s*)*)",
     re.IGNORECASE,
 )
-special_optional_regex = re.compile(special_optional_regex, re.IGNORECASE)
-tag_with_explanation_regex = re.compile(tag_with_explanation_regex, re.IGNORECASE)
+special_optional_regex: Pattern[str] = re.compile(
+    special_optional_regex_raw, re.IGNORECASE
+)
+tag_with_explanation_regex: Pattern[str] = re.compile(
+    tag_with_explanation_regex_raw, re.IGNORECASE
+)
 
-nodoctest_regex = re.compile(r'\s*(#+|%+|r"+|"+|\.\.)\s*nodoctest')
-optionaltag_regex = re.compile(r"^(\w|[.])+$")
-optionalfiledirective_regex = re.compile(
+no_doctest_regex: Pattern[str] = re.compile(r'\s*(#+|%+|r"+|"+|\.\.)\s*nodoctest')
+optional_tag_regex: Pattern[str] = re.compile(r"^(\w|[.])+$")
+optional_file_directive_regex: Pattern[str] = re.compile(
     r'\s*(#+|%+|r"+|"+|\.\.)\s*sage\.doctest: (.*)'
 )
 
@@ -167,7 +175,7 @@ def parse_optional_tags(
         ....:                     return_string_sans_tags=True)
         ({'scipy': None}, 'sage: #this is not \n....: import scipy', False)
     """
-    safe, literals, state = strip_string_literals(string)
+    safe, literals, _ = strip_string_literals(string)
     split = safe.split('\n', 1)
     if len(split) > 1:
         first_line, rest = split
@@ -231,7 +239,7 @@ def parse_optional_tags(
         return tags
 
 
-def parse_file_optional_tags(lines):
+def parse_file_optional_tags(lines) -> dict[str, str | None]:
     r"""
     Scan the first few lines for file-level doctest directives.
 
@@ -261,11 +269,11 @@ def parse_file_optional_tags(lines):
         ....:     parse_file_optional_tags(enumerate(f))
         {'xyz': None}
     """
-    tags = {}
+    tags: dict[str, str | None] = {}
     for line_count, line in lines:
-        if nodoctest_regex.match(line):
+        if no_doctest_regex.match(line):
             tags['not tested'] = None
-        if m := optionalfiledirective_regex.match(line):
+        if m := optional_file_directive_regex.match(line):
             file_tag_string = m.group(2)
             tags.update(parse_optional_tags('#' + file_tag_string))
         if line_count >= 10:
@@ -274,7 +282,7 @@ def parse_file_optional_tags(lines):
 
 
 @cached_function
-def _standard_tags():
+def _standard_tags() -> frozenset[str]:
     r"""
     Return the set of the names of all standard features.
 
@@ -321,7 +329,7 @@ def _tag_group(tag):
         return 'special'
 
 
-def unparse_optional_tags(tags, prefix='# '):
+def unparse_optional_tags(tags, prefix='# ') -> str:
     r"""
     Return a comment string that sets ``tags``.
 
@@ -596,7 +604,7 @@ def parse_tolerance(source, want):
     return want
 
 
-def pre_hash(s):
+def pre_hash(s) -> str:
     """
     Prepends a string with its length.