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add support for f-string and concatenated string mutations
Extends operator_string to mutate FormattedString and ConcatenatedString node types. F-strings now mutate the first non-empty FormattedStringText part, preserving whitespace. Concatenated strings are mutated through their individual string components. Fixes #439
1 parent b6c8b6b commit e8e9bb9

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5 files changed

+152
-97
lines changed

5 files changed

+152
-97
lines changed

‎mutmut/node_mutation.py‎

Lines changed: 43 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -57,6 +57,49 @@ def operator_string(
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continue
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yield node.with_changes(value=new_value)
5959

60+
elif isinstance(node, cst.FormattedString):
61+
# Mutate the first non-empty FormattedStringText part in f-strings
62+
for i, part in enumerate(node.parts):
63+
if isinstance(part, cst.FormattedStringText) and part.value.strip():
64+
# Apply mutations preserving leading whitespace
65+
old_value = part.value
66+
67+
# Preserve leading whitespace, mutate the rest
68+
stripped = old_value.lstrip()
69+
leading_ws = old_value[:len(old_value) - len(stripped)]
70+
71+
supported_str_mutations: list[Callable[[str], str]] = [
72+
lambda x: "XX" + x + "XX",
73+
lambda x: NON_ESCAPE_SEQUENCE.sub(lambda match: match.group(1).lower(), x),
74+
lambda x: NON_ESCAPE_SEQUENCE.sub(lambda match: match.group(1).upper(), x),
75+
]
76+
77+
for mut_func in supported_str_mutations:
78+
mutated_content = mut_func(stripped)
79+
if mutated_content == stripped:
80+
continue
81+
82+
new_value = leading_ws + mutated_content
83+
if new_value == old_value:
84+
continue
85+
86+
# Create a new part with the mutated value
87+
mutated_part = part.with_changes(value=new_value)
88+
# Replace the part in the parts list
89+
new_parts = [*node.parts[:i], mutated_part, *node.parts[i+1:]]
90+
yield node.with_changes(parts=new_parts)
91+
92+
# Only mutate the first non-empty text part
93+
break
94+
95+
elif isinstance(node, cst.ConcatenatedString):
96+
# ConcatenatedString nodes themselves don't need mutation; their SimpleString/FormattedString
97+
# children will be mutated directly by the mutation framework when it visits them.
98+
# Note: Currently, all parts of a concatenated string are mutated. Ideally, we would only
99+
# mutate the first non-empty part to reduce mutant count, but that would require changes
100+
# to the visitor pattern in file_mutation.py to skip certain children.
101+
return
102+
60103

61104
def operator_lambda(
62105
node: cst.Lambda

‎tests/e2e/snapshots/config.json‎

Lines changed: 8 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -1,15 +1,15 @@
11
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22
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‎tests/e2e/snapshots/mutate_only_covered_lines.json‎

Lines changed: 40 additions & 35 deletions
Original file line numberDiff line numberDiff line change
@@ -1,39 +1,44 @@
11
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3843
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3944
}

‎tests/e2e/snapshots/my_lib.json‎

Lines changed: 54 additions & 54 deletions
Original file line numberDiff line numberDiff line change
@@ -1,69 +1,69 @@
11
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22
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3-
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4-
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12-
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13-
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20-
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22-
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30-
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31-
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35-
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6868
}
6969
}

‎tests/test_mutation.py‎

Lines changed: 7 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -154,6 +154,13 @@ def mutated_module(source: str) -> str:
154154
('"FoO"', ['"XXFoOXX"', '"foo"', '"FOO"']),
155155
("'FoO'", ["'XXFoOXX'", "'foo'", "'FOO'"]),
156156
("u'FoO'", ["u'XXFoOXX'", "u'foo'", "u'FOO'"]),
157+
# f-strings - mutate the first non-empty text part
158+
('f"Hello {name}"', ['f"XXHello XX{name}"', 'f"hello {name}"', 'f"HELLO {name}"']),
159+
('f"Hello {x} World"', ['f"XXHello XX{x} World"', 'f"hello {x} World"', 'f"HELLO {x} World"']),
160+
('f"{x} World"', ['f"{x} XXWorldXX"', 'f"{x} world"', 'f"{x} WORLD"']),
161+
('f"{x}{y}"', []), # no text parts to mutate
162+
# concatenated strings - currently mutates all parts
163+
('"Hello " "World"', ['"XXHello XX" "World"', '"hello " "World"', '"HELLO " "World"', '"Hello " "XXWorldXX"', '"Hello " "world"', '"Hello " "WORLD"']),
157164
("10", "11"),
158165
("10.", "11.0"),
159166
("0o10", "9"),

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