From b719d85ccc330170e40b2617307a7e3b2a0bab14 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Sun, 19 Aug 2018 21:02:06 -0700
Subject: [PATCH] autodetect Python 3.6 on the basis of underscores (#461)

---
 README.md           | 8 ++++++++
 black.py            | 7 ++++++-
 tests/test_black.py | 4 ++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 96a24a1..9cb4f18 100644
--- a/README.md
+++ b/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)
 
diff --git a/black.py b/black.py
index c899bde..b65693e 100644
--- a/black.py
+++ b/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
diff --git a/tests/test_black.py b/tests/test_black.py
index 884d046..10d7f28 100644
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -722,6 +722,10 @@ class BlackTestCase(unittest.TestCase):
         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))
-- 
2.39.5