ENH: Add SIM107

Close issue #9

README.md

@@ -37,6 +37,7 @@ $ flake8 .
 * `SIM104`: Use 'yield from iterable' (introduced in Python 3.3, see [PEP 380](https://docs.python.org/3/whatsnew/3.3.html#pep-380))
 * `SIM105`: Use ['contextlib.suppress(...)'](https://docs.python.org/3/library/contextlib.html#contextlib.suppress) instead of try-except-pass
 * `SIM106`: Handle error-cases first
+* `SIM107`: Don't use `return` in try/except and finally.
 * `SIM201`: Use 'a != b' instead of 'not a == b'
 * `SIM202`: Use 'a == b' instead of 'not a != b'
 * `SIM203`: Use 'a not in b' instead of 'not (a in b)'
@@ -87,6 +88,33 @@ if a and b:
     c
 ```
 
+### SIM107
+
+```python
+# Bad: This example returns 3!
+def foo():
+    try:
+        1 / 0
+        return "1"
+    except:
+        return "2"
+    finally:
+        return "3"
+
+
+# Good
+def foo():
+    return_value = None
+    try:
+        1 / 0
+        return_value = "1"
+    except:
+        return_value = "2"
+    finally:
+        return_value = "3"  # you might also want to check if "except" happened
+    return return_value
+```
+
 ### SIM201
 
 ```python