diff -r 61d7aeb6aa1b Include/pymath.h
--- a/Include/pymath.h	Tue Aug 09 17:45:15 2016 -0500
+++ b/Include/pymath.h	Tue Aug 09 18:45:28 2016 -0700
@@ -55,6 +55,12 @@
 #define Py_MATH_E 2.7182818284590452354
 #endif
 
+/* Tau (2pi) to 40 digits, taken from tauday.com/tau-digits. */
+#ifndef Py_MATH_TAU
+#define Py_MATH_TAU 6.2831853071795864769252867665590057683943L
+#endif
+
+
 /* On x86, Py_FORCE_DOUBLE forces a floating-point number out of an x87 FPU
    register and into a 64-bit memory location, rounding from extended
    precision to double precision in the process.  On other platforms it does
diff -r 61d7aeb6aa1b Modules/mathmodule.c
--- a/Modules/mathmodule.c	Tue Aug 09 17:45:15 2016 -0500
+++ b/Modules/mathmodule.c	Tue Aug 09 18:45:28 2016 -0700
@@ -2144,6 +2144,7 @@
 
     PyModule_AddObject(m, "pi", PyFloat_FromDouble(Py_MATH_PI));
     PyModule_AddObject(m, "e", PyFloat_FromDouble(Py_MATH_E));
+    PyModule_AddObject(m, "tau", PyFloat_FromDouble(Py_MATH_TAU));  /* 2pi */
     PyModule_AddObject(m, "inf", PyFloat_FromDouble(m_inf()));
 #if !defined(PY_NO_SHORT_FLOAT_REPR) || defined(Py_NAN)
     PyModule_AddObject(m, "nan", PyFloat_FromDouble(m_nan()));