Report E741 for prohibited single-letter variables

Check for prohibited identifiers occuring to the lhs of an assignment
operator or after the 'as' keyword.

Closes #341

Author: mtmiller

CHANGES.txt

@@ -16,6 +16,7 @@ Changes:
 * Added check E275 for whitespace on `from ... import ...` lines; #489 / #491
 * Added W503 to the list of codes ignored by default ignore list; #498
 * Removed use of project level `.pep8` configuration file; #364
+* Added check E741 for using variables named 'l', 'O', or 'I'; #341
 
 Bugs:
 

docs/intro.rst

@@ -358,6 +358,8 @@ This is the current list of error and warning codes:
 +------------+----------------------------------------------------------------------+
 | E731       | do not assign a lambda expression, use a def                         |
 +------------+----------------------------------------------------------------------+
+| E741       | do not use variables named 'l', 'O', or 'I'                          |
++------------+----------------------------------------------------------------------+
 +------------+----------------------------------------------------------------------+
 | **E9**     | *Runtime*                                                            |
 +------------+----------------------------------------------------------------------+

pycodestyle.py

@@ -1150,6 +1150,37 @@ def comparison_type(logical_line, noqa):
         yield match.start(), "E721 do not compare types, use 'isinstance()'"
 
 
+def ambiguous_identifier(logical_line, tokens):
+    r"""Never use the characters 'l', 'O', or 'I' as variable names.
+
+    In some fonts, these characters are indistinguishable from the numerals
+    one and zero. When tempted to use 'l', use 'L' instead.
+
+    Okay: L = 0
+    E741: l = 0
+    """
+    idents_to_avoid = ('l', 'O', 'I')
+    prev_type, prev_text, prev_start, prev_end, __ = tokens[0]
+    for token_type, text, start, end, line in tokens[1:]:
+        ident = pos = None
+        # identifiers on the lhs of an assignment operator
+        if token_type == tokenize.OP and '=' in text:
+            if prev_text in idents_to_avoid:
+                ident = prev_text
+                pos = prev_start
+        # identifiers after 'as'
+        if prev_text == 'as':
+            if text in idents_to_avoid:
+                ident = text
+                pos = start
+        if ident:
+            yield pos, "E741 ambiguous variable name '%s'" % ident
+        prev_type = token_type
+        prev_text = text
+        prev_start = start
+        prev_end = end
+
+
 def python_3000_has_key(logical_line, noqa):
     r"""The {}.has_key() method is removed in Python 3: use the 'in' operator.