gh-36628: some simplifications in doctest/ folder (ruff C4)
    
use `ruff check --fix --select=C4 src/sage/doctest` to fix a few details

plus some manual tweaking afterwards

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #36628
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe

Author: Release Manager

src/sage/doctest/control.py

@@ -36,7 +36,6 @@
 import sys
 import time
 import json
-import re
 import shlex
 import types
 import sage.misc.flatten
@@ -66,6 +65,7 @@
 except ImportError:
     pass
 
+
 class DocTestDefaults(SageObject):
     """
     This class is used for doctesting the Sage doctest module.
@@ -145,7 +145,7 @@ def __init__(self, **kwds):
         # automatically anyway. However, this default is still used for
         # displaying user-defined optional tags and we don't want to see
         # the auto_optional_tags there.
-        self.optional = set(['sage']) | auto_optional_tags
+        self.optional = {'sage'} | auto_optional_tags
         self.hide = ''
         self.probe = ''
 
@@ -224,6 +224,7 @@ def skipdir(dirname):
         return True
     return False
 
+
 def skipfile(filename, tested_optional_tags=False, *,
              if_installed=False, log=None):
     """
@@ -317,8 +318,8 @@ def skipfile(filename, tested_optional_tags=False, *,
             return file_tag_string
 
     elif tested_optional_tags is not True:
-        extra = set(tag for tag in file_optional_tags
-                    if tag not in tested_optional_tags)
+        extra = {tag for tag in file_optional_tags
+                 if tag not in tested_optional_tags}
         if extra:
             file_tag_string = unparse_optional_tags(file_optional_tags, prefix='')
             if log:
@@ -445,7 +446,7 @@ def __init__(self, options, args):
         options.hidden_features = set()
         if isinstance(options.hide, str):
             if not len(options.hide):
-                options.hide = set([])
+                options.hide = set()
             else:
                 s = options.hide.lower()
                 options.hide = set(s.split(','))
@@ -455,12 +456,12 @@ def __init__(self, options, args):
             if 'all' in options.hide:
                 options.hide.discard('all')
                 from sage.features.all import all_features
-                feature_names = set([f.name for f in all_features() if not f.is_standard()])
+                feature_names = {f.name for f in all_features() if not f.is_standard()}
                 options.hide = options.hide.union(feature_names)
             if 'optional' in options.hide:
                 options.hide.discard('optional')
                 from sage.features.all import all_features
-                feature_names = set([f.name for f in all_features() if f.is_optional()])
+                feature_names = {f.name for f in all_features() if f.is_optional()}
                 options.hide = options.hide.union(feature_names)
 
         options.disabled_optional = set()
@@ -1085,7 +1086,7 @@ def sort_sources(self):
         """
         if self.options.nthreads > 1 and len(self.sources) > self.options.nthreads:
             self.log("Sorting sources by runtime so that slower doctests are run first....")
-            default = dict(walltime=0)
+            default = {'walltime': 0}
 
             def sort_key(source):
                 basename = source.basename
@@ -1153,7 +1154,7 @@ def run_doctests(self):
                     self.cleanup(False)
         else:
             self.log("No files to doctest")
-            self.reporter = DictAsObject(dict(error_status=0, stats={}))
+            self.reporter = DictAsObject({'error_status': 0, 'stats': {}})
 
     def cleanup(self, final=True):
         """
@@ -1315,9 +1316,9 @@ def run_val_gdb(self, testing=False):
                 flags = os.getenv("SAGE_MEMCHECK_FLAGS")
                 if flags is None:
                     flags = "--leak-resolution=high --leak-check=full --num-callers=25 "
-                    flags += '''--suppressions="%s" ''' % (os.path.join(SAGE_EXTCODE,"valgrind","pyalloc.supp"))
-                    flags += '''--suppressions="%s" ''' % (os.path.join(SAGE_EXTCODE,"valgrind","sage.supp"))
-                    flags += '''--suppressions="%s" ''' % (os.path.join(SAGE_EXTCODE,"valgrind","sage-additional.supp"))
+                    flags += '''--suppressions="%s" ''' % (os.path.join(SAGE_EXTCODE,"valgrind", "pyalloc.supp"))
+                    flags += '''--suppressions="%s" ''' % (os.path.join(SAGE_EXTCODE,"valgrind", "sage.supp"))
+                    flags += '''--suppressions="%s" ''' % (os.path.join(SAGE_EXTCODE,"valgrind", "sage-additional.supp"))
             elif opt.massif:
                 toolname = "massif"
                 flags = os.getenv("SAGE_MASSIF_FLAGS", "--depth=6 ")

src/sage/doctest/forker.py

@@ -687,10 +687,10 @@ def compiler(example):
                 # findlinestarts() returns pairs (index, lineno) where
                 # "index" is the index in the bytecode where the line
                 # number changes to "lineno".
-                linenumbers1 = set(lineno for (index, lineno)
-                                   in findlinestarts(code))
-                linenumbers2 = set(lineno for (index, lineno)
-                                   in findlinestarts(execcode))
+                linenumbers1 = {lineno for (index, lineno)
+                                in findlinestarts(code)}
+                linenumbers2 = {lineno for (index, lineno)
+                                in findlinestarts(execcode)}
                 if linenumbers1 != linenumbers2:
                     raise SyntaxError("doctest is not a single statement")
 
@@ -1726,7 +1726,7 @@ def serial_dispatch(self):
 
             with tempfile.TemporaryFile() as outtmpfile:
                 result = DocTestTask(source)(self.controller.options,
-                        outtmpfile, self.controller.logger)
+                                             outtmpfile, self.controller.logger)
                 outtmpfile.seek(0)
                 output = bytes_to_str(outtmpfile.read())
 
@@ -2334,7 +2334,7 @@ def save_result_output(self):
         try:
             self.result = self.result_queue.get(block=False)
         except Empty:
-            self.result = (0, DictAsObject(dict(err='noresult')))
+            self.result = (0, DictAsObject({'err': 'noresult'}))
         del self.result_queue
 
         self.outtmpfile.seek(0)
@@ -2536,17 +2536,17 @@ def __call__(self, options, outtmpfile=None, msgfile=None, result_queue=None):
         result = None
         try:
             runner = SageDocTestRunner(
-                    SageOutputChecker(),
-                    verbose=options.verbose,
-                    outtmpfile=outtmpfile,
-                    msgfile=msgfile,
-                    sage_options=options,
-                    optionflags=doctest.NORMALIZE_WHITESPACE | doctest.ELLIPSIS)
+                SageOutputChecker(),
+                verbose=options.verbose,
+                outtmpfile=outtmpfile,
+                msgfile=msgfile,
+                sage_options=options,
+                optionflags=doctest.NORMALIZE_WHITESPACE | doctest.ELLIPSIS)
             runner.basename = self.source.basename
             runner.filename = self.source.path
             N = options.file_iterations
-            results = DictAsObject(dict(walltime=[], cputime=[],
-                                        err=None, walltime_skips=0))
+            results = DictAsObject({'walltime': [], 'cputime': [],
+                                    'err': None, 'walltime_skips': 0})
 
             # multiprocessing.Process instances don't run exit
             # functions, so we run the functions added by doctests
@@ -2571,7 +2571,7 @@ def __call__(self, options, outtmpfile=None, msgfile=None, result_queue=None):
         except BaseException:
             exc_info = sys.exc_info()
             tb = "".join(traceback.format_exception(*exc_info))
-            result = (0, DictAsObject(dict(err=exc_info[0], tb=tb)))
+            result = (0, DictAsObject({'err': exc_info[0], 'tb': tb}))
 
         if result_queue is not None:
             result_queue.put(result, False)

src/sage/doctest/parsing.py

@@ -94,7 +94,7 @@ def fake_RIFtol(*args):
 ansi_escape_sequence = re.compile(r'(\x1b[@-Z\\-~]|\x1b\[.*?[@-~]|\x9b.*?[@-~])')
 
 special_optional_regex = 'arb216|arb218|py2|long time|not implemented|not tested|known bug'
-tag_with_explanation_regex = fr'((?:\w|[.])+)\s*(?:\((.*?)\))?'
+tag_with_explanation_regex = r'((?:\w|[.])+)\s*(?:\((.*?)\))?'
 optional_regex = re.compile(fr'(?P<cmd>{special_optional_regex})\s*(?:\((?P<cmd_explanation>.*?)\))?|'
                             fr'[^ a-z]\s*(optional|needs)(?:\s|[:-])*(?P<tags>(?:(?:{tag_with_explanation_regex})\s*)*)',
                             re.IGNORECASE)
@@ -470,7 +470,7 @@ def update_optional_tags(line, tags=None, *, add_tags=None, remove_tags=None, fo
         |                                                V       V       V       V       V   V   v           v                   v                                       v
         |    sage: # optional - magma, needs sage.symbolic
     """
-    if not (m := re.match('( *sage: *)(.*)', line)):
+    if not re.match('( *sage: *)(.*)', line):
         raise ValueError(f'line must start with a sage: prompt, got: {line}')
 
     current_tags, line_sans_tags, is_persistent = parse_optional_tags(line.rstrip(), return_string_sans_tags=True)
@@ -1140,8 +1140,8 @@ def update_tag_counts(optional_tags):
 
         def check_and_clear_tag_counts():
             if (num_examples := tag_count_within_block['']) >= 4:
-                if overused_tags := set(tag for tag, count in tag_count_within_block.items()
-                                        if tag and count >= num_examples):
+                if overused_tags := {tag for tag, count in tag_count_within_block.items()
+                                     if tag and count >= num_examples}:
                     overused_tags.update(persistent_optional_tags)
                     overused_tags.difference_update(self.file_optional_tags)
                     suggested = unparse_optional_tags(overused_tags, prefix='sage: # ')
@@ -1210,10 +1210,10 @@ def check_and_clear_tag_counts():
                             continue
 
                     if self.optional_tags is not True:
-                        extra = set(tag
-                                    for tag in optional_tags
-                                    if (tag not in self.optional_tags
-                                        and tag not in available_software))
+                        extra = {tag
+                                 for tag in optional_tags
+                                 if (tag not in self.optional_tags
+                                     and tag not in available_software)}
                         if extra:
                             if any(tag in external_software for tag in extra):
                                 # never probe "external" software

src/sage/doctest/reporting.py

@@ -116,7 +116,7 @@ def __init__(self, controller):
             sage: DTR = DocTestReporter(DC)
         """
         self.controller = controller
-        self.postscript = dict(lines=[], cputime=0, walltime=0)
+        self.postscript = {"lines": [], "cputime": 0, "walltime": 0}
         self.sources_completed = 0
         self.stats = {}
         self.error_status = 0
@@ -408,7 +408,7 @@ def report(self, source, timeout, return_code, results, output, pid=None):
                 ntests, result_dict = results
             except (TypeError, ValueError):
                 ntests = 0
-                result_dict = DictAsObject(dict(err='badresult'))
+                result_dict = DictAsObject({"err": 'badresult'})
             if timeout:
                 fail_msg = "Timed out"
                 if ntests > 0:
@@ -429,7 +429,7 @@ def report(self, source, timeout, return_code, results, output, pid=None):
                 log(output)
                 log("*"*70)
                 postscript['lines'].append(cmd + "  # %s" % fail_msg)
-                stats[basename] = dict(failed=True, walltime=1e6, ntests=ntests)
+                stats[basename] = {"failed": True, "walltime": 1e6, "ntests": ntests}
                 if not the_baseline_stats.get('failed', False):
                     self.error_status |= 4
             elif return_code:
@@ -445,7 +445,7 @@ def report(self, source, timeout, return_code, results, output, pid=None):
                 log(output)
                 log("*"*70)
                 postscript['lines'].append(cmd + "  # %s" % fail_msg)
-                stats[basename] = dict(failed=True, walltime=1e6, ntests=ntests)
+                stats[basename] = {"failed": True, "walltime": 1e6, "ntests": ntests}
                 if not the_baseline_stats.get('failed', False):
                     self.error_status |= (8 if return_code > 0 else 16)
             else:
@@ -501,9 +501,9 @@ def report(self, source, timeout, return_code, results, output, pid=None):
                     if hasattr(result_dict, 'tb'):
                         log(result_dict.tb)
                     if hasattr(result_dict, 'walltime'):
-                        stats[basename] = dict(failed=True, walltime=wall, ntests=ntests)
+                        stats[basename] = {"failed": True, "walltime": wall, "ntests": ntests}
                     else:
-                        stats[basename] = dict(failed=True, walltime=1e6, ntests=ntests)
+                        stats[basename] = {"failed": True, "walltime": 1e6, "ntests": ntests}
                     self.error_status |= 64
                 if result_dict.err is None or result_dict.err == 'tab':
                     f = result_dict.failures
@@ -515,16 +515,16 @@ def report(self, source, timeout, return_code, results, output, pid=None):
                         if not the_baseline_stats.get('failed', False):
                             self.error_status |= 1
                     if f or result_dict.err == 'tab':
-                        stats[basename] = dict(failed=True, walltime=wall, ntests=ntests)
+                        stats[basename] = {"failed": True, "walltime": wall, "ntests": ntests}
                     else:
-                        stats[basename] = dict(walltime=wall, ntests=ntests)
+                        stats[basename] = {"walltime": wall, "ntests": ntests}
                     postscript['cputime'] += cpu
                     postscript['walltime'] += wall
 
                     try:
                         optionals = result_dict.optionals
                     except AttributeError:
-                        optionals = dict()
+                        optionals = {}
                     for tag in sorted(optionals):
                         nskipped = optionals[tag]
                         if tag == "long time":

src/sage/doctest/sources.py

@@ -233,7 +233,7 @@ def _process_doc(self, doctests, doc, namespace, start):
         new_doctests = self.parse_docstring(docstring, namespace, start)
         sig_on_count_doc_doctest = "sig_on_count() # check sig_on/off pairings (virtual doctest)\n"
         for dt in new_doctests:
-            if len(dt.examples) > 0 and not (hasattr(dt.examples[-1],'sage_source')
+            if len(dt.examples) > 0 and not (hasattr(dt.examples[-1], 'sage_source')
                                              and dt.examples[-1].sage_source == sig_on_count_doc_doctest):
                 # Line number refers to the end of the docstring
                 sigon = doctest.Example(sig_on_count_doc_doctest, "0\n", lineno=docstring.count("\n"))
@@ -305,7 +305,7 @@ def _create_doctests(self, namespace, tab_okay=None):
             False
         """
         if tab_okay is None:
-            tab_okay = isinstance(self,TexSource)
+            tab_okay = isinstance(self, TexSource)
         self._init()
         self.line_shift = 0
         self.parser = SageDocTestParser(self.options.optional,
@@ -371,9 +371,9 @@ def _create_doctests(self, namespace, tab_okay=None):
         if unparsed_doc:
             self._process_doc(doctests, doc, namespace, start)
 
-        extras = dict(tab=not tab_okay and tab_locations,
-                      line_number=contains_line_number,
-                      optionals=self.parser.optionals)
+        extras = {"tab": not tab_okay and tab_locations,
+                  "line_number": contains_line_number,
+                  "optionals": self.parser.optionals}
         if self.options.randorder is not None and self.options.randorder is not False:
             # we want to randomize even when self.randorder = 0
             random.seed(self.options.randorder)
@@ -569,13 +569,13 @@ def __init__(self, path, options):
             base, ext = os.path.splitext(path)
         valid_code_ext = ('.py', '.pyx', '.pxd', '.pxi', '.sage', '.spyx')
         if ext in valid_code_ext:
-            self.__class__ = dynamic_class('PythonFileSource',(FileDocTestSource,PythonSource))
+            self.__class__ = dynamic_class('PythonFileSource', (FileDocTestSource, PythonSource))
             self.encoding = "utf-8"
         elif ext == '.tex':
-            self.__class__ = dynamic_class('TexFileSource',(FileDocTestSource,TexSource))
+            self.__class__ = dynamic_class('TexFileSource', (FileDocTestSource, TexSource))
             self.encoding = "utf-8"
         elif ext == '.rst' or ext == '.rst.txt':
-            self.__class__ = dynamic_class('RestFileSource',(FileDocTestSource,RestSource))
+            self.__class__ = dynamic_class('RestFileSource', (FileDocTestSource, RestSource))
             self.encoding = "utf-8"
         else:
             valid_ext = ", ".join(valid_code_ext + ('.tex', '.rst', '.rst.txt'))
@@ -955,6 +955,7 @@ def parse_docstring(self, docstring, namespace, start):
         return [self.parser.get_doctest(docstring, namespace, str(self.qualified_name),
                                         self.printpath, start + 1)]
 
+
 class PythonSource(SourceLanguage):
     """
     This class defines the functions needed for the extraction of doctests from python sources.
@@ -1252,6 +1253,7 @@ def _neutralize_doctests(self, reindent):
                 neutralized.append(" "*reindent + line)
         return "".join(neutralized)
 
+
 class TexSource(SourceLanguage):
     """
     This class defines the functions needed for the extraction of
@@ -1628,6 +1630,7 @@ def parse_docstring(self, docstring, namespace, start):
                                                 self.printpath, start + 1)
         return [outer_doctest] + inner_doctests
 
+
 class DictAsObject(dict):
     """
     A simple subclass of dict that inserts the items from the initializing dictionary into attributes.

src/sage/doctest/util.py

@@ -81,7 +81,7 @@ def dict_difference(self, other):
         sage: dict_difference(D2.__dict__, D1.__dict__)
         {'foobar': 'hello', 'timeout': 100}
     """
-    D = dict()
+    D = {}
     for k, v in self.items():
         try:
             if other[k] == v:
@@ -275,8 +275,8 @@ def start(self):
             sage: D.start(); D.set
             set()
         """
-        self.set = set([])
-        self.got = set([])
+        self.set = set()
+        self.got = set()
 
     def __getitem__(self, name):
         """