diff --git a/src/runtime/python/README.md b/src/runtime/python/README.md
index 504a5c529..4ffd9cca4 100644
--- a/src/runtime/python/README.md
+++ b/src/runtime/python/README.md
@@ -1,5 +1,9 @@
 # Python bindings to C runtime
 
+## Compatibility
+
+These bindings do not support Python 2. Python 3 is assumed in all these instructions.
+
 ## Pre-requisites
 
 1. You must have installed the PGF C runtime (see `../c/README.md`)
@@ -26,10 +30,3 @@ pytest
 ```
 
 Requires: `pip install pytest`
-
-
-## Debugging
-
-valgrind --leak-check=full --show-leak-kinds=all pytest 2> valgrind.txt
-
-valgrind --leak-check=full --show-leak-kinds=all -- python -c 'import pgf; s = pgf.readType("A -> B"); print(s)' 2> valgrind.txt
diff --git a/src/runtime/python/expr.h b/src/runtime/python/expr.h
index 0fa1e90ea..e9d11d677 100644
--- a/src/runtime/python/expr.h
+++ b/src/runtime/python/expr.h
@@ -13,7 +13,7 @@ typedef struct {
     PyObject *exprs; // PyTupleObject of ExprObject
 } TypeObject;
 
-PyTypeObject pgf_TypeType;
+extern PyTypeObject pgf_TypeType;
 
 // typedef struct {
 //     PyObject_HEAD
@@ -72,14 +72,14 @@ typedef struct {
     ExprObject *expr;
 } ExprImplArgObject;
 
-PyTypeObject pgf_ExprType;
-PyTypeObject pgf_ExprAbsType;
-PyTypeObject pgf_ExprAppType;
-PyTypeObject pgf_ExprLitType;
-PyTypeObject pgf_ExprMetaType;
-PyTypeObject pgf_ExprFunType;
-PyTypeObject pgf_ExprVarType;
-PyTypeObject pgf_ExprTypedType;
-PyTypeObject pgf_ExprImplArgType;
+extern PyTypeObject pgf_ExprType;
+extern PyTypeObject pgf_ExprAbsType;
+extern PyTypeObject pgf_ExprAppType;
+extern PyTypeObject pgf_ExprLitType;
+extern PyTypeObject pgf_ExprMetaType;
+extern PyTypeObject pgf_ExprFunType;
+extern PyTypeObject pgf_ExprVarType;
+extern PyTypeObject pgf_ExprTypedType;
+extern PyTypeObject pgf_ExprImplArgType;
 
 #endif // PYPGF_EXPR_H_
diff --git a/src/runtime/python/ffi.h b/src/runtime/python/ffi.h
index 1d3ca3f25..823f2ff71 100644
--- a/src/runtime/python/ffi.h
+++ b/src/runtime/python/ffi.h
@@ -12,7 +12,7 @@ typedef struct {
     PgfRevision revision;
 } PGFObject;
 
-PyObject *PGFError;
+extern PyObject *PGFError;
 PgfExnType handleError(PgfExn err);
 
 PgfText *CString_AsPgfText(const char *s, size_t size);
@@ -29,7 +29,7 @@ void FreePgfText(PgfText *txt);
 void FreeHypos(PgfTypeHypo *hypos, Py_ssize_t n_hypos);
 void FreePgfPrintContext(PgfPrintContext *ctxt);
 
-PgfUnmarshaller unmarshaller;
-PgfMarshaller marshaller;
+extern PgfUnmarshaller unmarshaller;
+extern PgfMarshaller marshaller;
 
 #endif // PYPGF_FFI_H_
diff --git a/src/runtime/python/transactions.h b/src/runtime/python/transactions.h
index af70d5e29..61e3692e7 100644
--- a/src/runtime/python/transactions.h
+++ b/src/runtime/python/transactions.h
@@ -12,7 +12,7 @@ typedef struct {
     PgfRevision revision; // transient branch
 } TransactionObject;
 
-PyTypeObject pgf_TransactionType;
+extern PyTypeObject pgf_TransactionType;
 
 PyObject *PGF_checkoutBranch(PGFObject *self, PyObject *args);