Report E731 for lambda assignment, return E704 for one-liner def instead of E701; issue #277

Author: florentx

docs/intro.rst

@@ -305,6 +305,8 @@ This is the current list of error and warning codes:
 +----------+----------------------------------------------------------------------+
 | E703     | statement ends with a semicolon                                      |
 +----------+----------------------------------------------------------------------+
+| E704     | multiple statements on one line (def)                                |
++----------+----------------------------------------------------------------------+
 | E711 (^) | comparison to None should be 'if cond is None:'                      |
 +----------+----------------------------------------------------------------------+
 | E712 (^) | comparison to True should be 'if cond is True:' or 'if cond:'        |
@@ -315,6 +317,8 @@ This is the current list of error and warning codes:
 +----------+----------------------------------------------------------------------+
 | E721     | do not compare types, use 'isinstance()'                             |
 +----------+----------------------------------------------------------------------+
+| E731     | do not assign a lambda expression, use a def                         |
++----------+----------------------------------------------------------------------+
 +----------+----------------------------------------------------------------------+
 | **E9**   | *Runtime*                                                            |
 +----------+----------------------------------------------------------------------+

pep8.py

@@ -834,6 +834,9 @@ def compound_statements(logical_line):
     on the same line, never do this for multi-clause statements.
     Also avoid folding such long lines!
 
+    Always use a def statement instead of an assignment statement that
+    binds a lambda expression directly to a name.
+
     Okay: if foo == 'blah':\n    do_blah_thing()
     Okay: do_one()
     Okay: do_two()
@@ -847,20 +850,26 @@ def compound_statements(logical_line):
     E701: try: something()
     E701: finally: cleanup()
     E701: if foo == 'blah': one(); two(); three()
-
     E702: do_one(); do_two(); do_three()
     E703: do_four();  # useless semicolon
+    E704: def f(x): return 2*x
+    E731: f = lambda x: 2*x
     """
     line = logical_line
     last_char = len(line) - 1
     found = line.find(':')
     while -1 < found < last_char:
         before = line[:found]
-        if (before.count('{') <= before.count('}') and  # {'a': 1} (dict)
-            before.count('[') <= before.count(']') and  # [1:2] (slice)
-            before.count('(') <= before.count(')') and  # (Python 3 annotation)
-                not LAMBDA_REGEX.search(before)):       # lambda x: x
-            yield found, "E701 multiple statements on one line (colon)"
+        if ((before.count('{') <= before.count('}') and   # {'a': 1} (dict)
+             before.count('[') <= before.count(']') and   # [1:2] (slice)
+             before.count('(') <= before.count(')'))):    # (annotation)
+            if LAMBDA_REGEX.search(before):
+                yield 0, "E731 do not assign a lambda expression, use a def"
+                break
+            if before.startswith('def '):
+                yield 0, "E704 multiple statements on one line (def)"
+            else:
+                yield found, "E701 multiple statements on one line (colon)"
         found = line.find(':', found + 1)
     found = line.find(';')
     while -1 < found:

testsuite/E22.py

@@ -135,8 +135,8 @@ def squares(n):
 negative = -1
 spam(-1)
 -negative
-lambda *args, **kw: (args, kw)
-lambda a, b=h[:], c=0: (a, b, c)
+func1(lambda *args, **kw: (args, kw))
+func2(lambda a, b=h[:], c=0: (a, b, c))
 if not -5 < x < +5:
     print >>sys.stderr, "x is out of range."
 print >> sys.stdout, "x is an integer."

testsuite/E70.py

@@ -1,13 +1,20 @@
-#: E701
+#: E701:1:5
 if a: a = False
-#: E701
+#: E701:1:40
 if not header or header[:6] != 'bytes=': return
-#: E702
+#: E702:1:10
 a = False; b = True
-#: E702
+#: E702:1:17
 import bdist_egg; bdist_egg.write_safety_flag(cmd.egg_info, safe)
-#: E703
+#: E703:1:13
 import shlex;
-#: E702 E703
+#: E702:1:9 E703:1:23
 del a[:]; a.append(42);
+#: E704:1:1
+def f(x): return 2
+#: E704:1:1 E226:1:19
+def f(x): return 2*x
+#: E704:2:5 E226:2:23
+while all is round:
+    def f(x): return 2*x
 #:

testsuite/E73.py

@@ -0,0 +1,7 @@
+#: E731:1:1
+f = lambda x: 2 * x
+#: E731:1:1 E226:1:16
+f = lambda x: 2*x
+#: E731:2:5
+while False:
+    this = lambda y, z: 2 * x