Add "set discard" check (#74):

When removing keys from a `set`, you can use `.remove()`, which assumes the
key already exists. If you just want to remove the key regardless of
whether it is already in the set or not, use `.discard()` instead.

One potential case that this check does not pick up on is the following:

```python
try:
    nums.remove(123)
except KeyError:
    pass
```

Since `.remove()` will throw a `KeyError` if the key doesn't exist, a valid
way of silencing this would be the above code. This is identical to the "with
suppress" check, and I don't like the interaction that all of these checks
are having with it. This also means that the following code should be checked:

```python
with suppress(KeyError):
    nums.remove(123)
```

Though adding 3 different styles for the same check is a lot of work, and
requires additional maintainence.

Also, this check (and many others) do not pick up on member expressions, such
as `x.y.z.remove(123)`. This will be addressed soon.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "refurb"
-version = "1.3.0"
+version = "1.4.0"
 description = "A tool for refurbish and modernize Python codebases"
 authors = ["dosisod"]
 license = "GPL-3.0-only"

refurb/checks/builtin/set_discard.py

@@ -0,0 +1,69 @@
+from dataclasses import dataclass
+
+from mypy.nodes import (
+    Block,
+    CallExpr,
+    ComparisonExpr,
+    ExpressionStmt,
+    IfStmt,
+    MemberExpr,
+    NameExpr,
+    Var,
+)
+
+from refurb.error import Error
+
+
+@dataclass
+class ErrorInfo(Error):
+    """
+    If you want to remove a value from a set regardless of whether it exists or
+    not, use the `discard()` method instead of `remove()`:
+
+    Bad:
+
+    ```
+    nums = set((123, 456))
+
+    if 123 in nums:
+        nums.remove(123)
+    ```
+
+    Good:
+
+    ```
+    nums = set((123, 456))
+
+    nums.discard(123)
+    ```
+    """
+
+    code = 132
+    msg: str = "Replace `if x in s: s.remove(x)` with `s.discard(x)`"
+
+
+def check(node: IfStmt, errors: list[Error]) -> None:
+    match node:
+        case IfStmt(
+            expr=[ComparisonExpr(operators=["in"], operands=[lhs, rhs])],
+            body=[
+                Block(
+                    body=[
+                        ExpressionStmt(
+                            expr=CallExpr(
+                                callee=MemberExpr(
+                                    expr=NameExpr(node=Var(type=ty)) as expr,
+                                    name="remove",
+                                ),
+                                args=[arg],
+                            )
+                        )
+                    ]
+                )
+            ],
+        ) if (
+            str(lhs) == str(arg)
+            and str(rhs) == str(expr)
+            and str(ty).startswith("builtins.set[")
+        ):
+            errors.append(ErrorInfo(node.line, node.column))

test/data/err_132.py

@@ -0,0 +1,34 @@
+s = set()
+
+# these should match
+
+if "x" in s:
+    s.remove("x")
+
+# these should not
+
+if "x" in s:
+    s.remove("y")
+
+s.discard("x")
+
+s2 = set()
+
+if "x" in s:
+    s2.remove("x")
+
+if "x" in s:
+    s.remove("x")
+    print("removed item")
+
+class Container:
+    def remove(self, item) -> None:
+        return
+
+    def __contains__(self, other) -> bool:
+        return True
+
+c = Container()
+
+if "x" in c:
+    c.remove("x")

test/data/err_132.txt

@@ -0,0 +1 @@
+test/data/err_132.py:5:1 [FURB132]: Replace `if x in s: s.remove(x)` with `s.discard(x)`