Python-style "x in xs" testing in Scala using implicit conversion and duck typing. Sinful but convenient.

x-in-xs.txt

scala> implicit class WithInMethod(obj: Any) {
     |   def in(a: {def contains(e: Any): Boolean}): Boolean = a.contains(obj)
     | }
warning: there were 1 feature warnings; re-run with -feature for details
defined class WithInMethod

scala> "foo" in List("1", "2")
res3: Boolean = false

scala> "foo" in List("foo", "bar")
res4: Boolean = true

scala> "foo" in Map("1" -> 1, "2" -> 2)
res5: Boolean = false

scala> "foo" in Map("foo" -> 1, "bar" -> 2)
res6: Boolean = true

x-not-in-xs.txt

scala> implicit class WithInAndNotMethods(obj: Any) {
     |   def in(a: {def contains(e: Any): Boolean}): Boolean = a.contains(obj)
     |   def not(f: Any => Boolean): Boolean = !f(obj)
     | }
warning: there were 1 feature warnings; re-run with -feature for details
defined class WithInAndNotMethods

scala> def in(a: {def contains(e: Any): Boolean}): Any => Boolean = a.contains _
warning: there were 2 feature warnings; re-run with -feature for details
in: (a: AnyRef{def contains(e: Any): Boolean})Any => Boolean

scala> "foo" in List("foo", "bar")
res0: Boolean = true

scala> "foo" in List("a", "b")
res1: Boolean = false

scala> "foo" not in List("foo", "bar")
<console>:10: error: type mismatch;
 found   : Any => Boolean
 required: Boolean
              "foo" not in List("foo", "bar")
                        ^

scala> "foo" not in (List("foo", "bar")) // shame, we have to add parentheses here
res3: Boolean = false

scala> "foo" not in (List("a", "b"))
res4: Boolean = true