setup: only allow Python 3 from a source checkout (issue5804) People are running `pip install Mercurial` with Python 3 and that is working because not everything performs a Python version compatibility check. Modern versions of pip do recognize the "python_requires" keyword (https://packaging.python.org/tutorials/distributing-packages/#python-requires) which we set if using setuptools. But this isn't set nor recognized everywhere. To prevent people from accidentally installing Mercurial with Python 3 until Python 3 is officially supported, have setup.py fail when run with Python 3. But don't fail if we're running from a source checkout, as we don't want to anger Mercurial developers hacking on Python 3 nor Mercurial's test automation running from source checkouts. People running setup.py from source checkouts could still fall through a Python 3 crack. But at least the `pip install Mercurial` attempt will get nipped in the bud.

--- a/setup.py	Wed Feb 21 16:51:09 2018 -0500
+++ b/setup.py	Fri Feb 23 17:57:04 2018 -0800
@@ -67,6 +67,26 @@
     printf(error, file=sys.stderr)
     sys.exit(1)
 
+# We don't yet officially support Python 3. But we want to allow developers to
+# hack on. Detect and disallow running on Python 3 by default. But provide a
+# backdoor to enable working on Python 3.
+if sys.version_info[0] != 2:
+    badpython = True
+
+    # Allow Python 3 from source checkouts.
+    if os.path.isdir('.hg'):
+        badpython = False
+
+    if badpython:
+        error = """
+Mercurial only supports Python 2.7.
+Python {py} detected.
+Please re-run with Python 2.7.
+""".format(py=sys.version_info)
+
+        printf(error, file=sys.stderr)
+        sys.exit(1)
+
 # Solaris Python packaging brain damage
 try:
     import hashlib