teyit assumes space-indented source

[nega0]

teyit mishandles tab-indented source. Yeah, I know, tabs...

Given the following test file:

import unittest
# indentation is tabs
class foo(unittest.TestCase):
	def testBar(self):
		print("testing")
		self.assertTrue("s" in "strings", "yay")
		print("testing done")

and the following diff against teyit.py (for illustrative purposes):

diff --git a/teyit.py b/teyit.py
index bfc9f59..a2d52aa 100644
--- a/teyit.py
+++ b/teyit.py
@@ -267,6 +267,8 @@ def rewrite_source(source, *, blacklist=frozenset()):
         return source, []
 
     original_source = source
+    print(source)
+    print("calling astparse()")
     tree = ast.parse(source)
     rewriter = _AssertRewriter(blacklist=blacklist)
     rewriter.visit(tree)
@@ -298,6 +300,7 @@ def rewrite_source(source, *, blacklist=frozenset()):
 def refactor_until_deterministic(source, blacklist=frozenset(), *, max=5):
     refactors = []
     for iteration in range(max):
+        print(f"{iteration=}")
         source, _refactors = rewrite_source(source, blacklist=blacklist)
         if len(_refactors) == 0:
             break

We get the following output

$ python3 teyit.py --show-stats t.py
iteration=0
import unittest

class foo(unittest.TestCase):
	def testBar(self):
		print("testing")
		self.assertTrue("s" in "strings", "yay")
		print("testing done")

calling astparse()
iteration=1
import unittest

class foo(unittest.TestCase):
	def testBar(self):
		print("testing")
  self.assertIn("s", "strings", "yay")
		print("testing done")

calling astparse()
Traceback (most recent call last):
  File "/Users/nega/Downloads/teyit/teyit.py", line 404, in <module>
    main()
  File "/Users/nega/Downloads/teyit/teyit.py", line 400, in main
    raise SystemExit(_refactor_files(**vars(options)))
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nega/Downloads/teyit/teyit.py", line 357, in _refactor_files
    if len(refactors := _refactor_file(path)) > 0:
                        ^^^^^^^^^^^^^^^^^^^^
  File "/Users/nega/Downloads/teyit/teyit.py", line 345, in _refactor_file
    refactored_source, refactors = refactor_until_deterministic(
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nega/Downloads/teyit/teyit.py", line 304, in refactor_until_deterministic
    source, _refactors = rewrite_source(source, blacklist=blacklist)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nega/Downloads/teyit/teyit.py", line 272, in rewrite_source
    tree = ast.parse(source)
           ^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.11/3.11.4_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<unknown>", line 6
    self.assertIn("s", "strings", "yay")
                                        ^
IndentationError: unindent does not match any outer indentation level

It looks like teyit is replacing "tab stops" with single spaces. Indeed that replacement is in as_source(). The column offset of our node is recorded, and the new code we're injecting is then indented by that many spaces (instead of that many "indent characters"). Not being that familiar with teyit or Python's ast, I don't see a clean way of getting the "indent character" into as_source().