Fix an issue with _graph_info (#1966)

_graph_info could contain non POD which broke things particularly in the presence of extensions

metaflow/flowspec.py

@@ -17,6 +17,7 @@
 )
 from .graph import FlowGraph
 from .unbounded_foreach import UnboundedForeachInput
+from .util import to_pod
 from .metaflow_config import INCLUDE_FOREACH_STACK, MAXIMUM_FOREACH_VALUE_CHARS
 
 # For Python 3 compatibility
@@ -201,7 +202,7 @@ def _set_constants(self, graph, kwargs):
             "decorators": [
                 {
                     "name": deco.name,
-                    "attributes": deco.attributes,
+                    "attributes": to_pod(deco.attributes),
                     "statically_defined": deco.statically_defined,
                 }
                 for deco in flow_decorators(self)

metaflow/graph.py

@@ -3,6 +3,9 @@
 import re
 
 
+from .util import to_pod
+
+
 def deindent_docstring(doc):
     if doc:
         # Find the indent to remove from the docstring. We consider the following possibilities:
@@ -72,7 +75,6 @@ def _expr_str(self, expr):
         return "%s.%s" % (expr.value.id, expr.attr)
 
     def _parse(self, func_ast):
-
         self.num_args = len(func_ast.args.args)
         tail = func_ast.body[-1]
 
@@ -262,7 +264,6 @@ def node_specs():
         )
 
     def output_steps(self):
-
         steps_info = {}
         graph_structure = []
 
@@ -286,7 +287,7 @@ def node_to_dict(name, node):
                 "decorators": [
                     {
                         "name": deco.name,
-                        "attributes": deco.attributes,
+                        "attributes": to_pod(deco.attributes),
                         "statically_defined": deco.statically_defined,
                     }
                     for deco in node.decorators

metaflow/util.py

@@ -426,6 +426,25 @@ def is_within_directory(abs_directory, target):
     tar.extractall(path, members, numeric_owner=numeric_owner)
 
 
+def to_pod(value):
+    """
+    Convert a python object to plain-old-data (POD) format.
+
+    Parameters
+    ----------
+    value : Any
+        Value to convert to POD format. The value can be a string, number, list,
+        dictionary, or a nested structure of these types.
+    """
+    if isinstance(value, (str, int, float)):
+        return value
+    if isinstance(value, dict):
+        return {to_pod(k): to_pod(v) for k, v in value.items()}
+    if isinstance(value, (list, set, tuple)):
+        return [to_pod(v) for v in value]
+    return str(value)
+
+
 if sys.version_info[:2] > (3, 5):
     from metaflow._vendor.packaging.version import parse as version_parse
 else: