autodetect Python 3.6 on the basis of underscores (#461)

README.md

@@ -371,6 +371,12 @@ human-readable strings"](https://stackoverflow.com/a/56190)), you can
 pass `--skip-string-normalization` on the command line.  This is meant as
 an adoption helper, avoid using this for new projects.
 
+### Numeric literals
+
+*Black* standardizes all numeric literals to use lowercase letters: `0xab`
+instead of `0XAB` and `1e10` instead of `1E10`. In Python 3.6+, *Black*
+adds underscores to long numeric literals to aid readability: `100000000`
+becomes `100_000_000`.
 
 ### Line breaks & binary operators
 
@@ -843,6 +849,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 ### 18.8b0
 
+* code with `_` in numeric literals is recognized as Python 3.6+ (#461)
+
 * numeric literals are now normalized to include `_` separators on Python 3.6+ code
   (#452)
 

black.py

@@ -2891,7 +2891,8 @@ def is_python36(node: Node) -> bool:
     """Return True if the current file is using Python 3.6+ features.
 
     Currently looking for:
-    - f-strings; and
+    - f-strings;
+    - underscores in numeric literals; and
     - trailing commas after * or ** in function signatures and calls.
     """
     for n in node.pre_order():
@@ -2900,6 +2901,10 @@ def is_python36(node: Node) -> bool:
             if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
                 return True
 
+        elif n.type == token.NUMBER:
+            if "_" in n.value:  # type: ignore
+                return True
+
         elif (
             n.type in {syms.typedargslist, syms.arglist}
             and n.children

tests/test_black.py

@@ -722,6 +722,10 @@ def test_is_python36(self) -> None:
         self.assertTrue(black.is_python36(node))
         node = black.lib2to3_parse("def f(*, arg): f'string'\n")
         self.assertTrue(black.is_python36(node))
+        node = black.lib2to3_parse("123_456\n")
+        self.assertTrue(black.is_python36(node))
+        node = black.lib2to3_parse("123456\n")
+        self.assertFalse(black.is_python36(node))
         source, expected = read_data("function")
         node = black.lib2to3_parse(source)
         self.assertTrue(black.is_python36(node))