py2: *: Emulate `raise from` by hand

`raise from` is invalid syntax on python2, but it still makes sense to
preserve error cause because pygolang can handle and display that even
on py2: nexedi/pygolang@bb9a94c3 .

amari/__init__.py

@@ -55,7 +55,7 @@ def connect(ctx, wsuri):  # -> Conn
         # FIXME handle ctx cancel  (but it won't stuck forever due to ._ws own timeout)
         ws.connect(wsuri)
     except Exception as ex:
-        raise ConnError("connect") from ex
+        raise_from(ConnError("connect"), ex)
     return Conn(ws, wsuri)
 
 # Conn represents WebSocket connection to a service.
@@ -86,7 +86,7 @@ class Conn:
                 raise ValueError("unexpected welcome message: %s" % msg0)
         except Exception as ex:
             ws.close()
-            raise ConnError("handshake") from ex
+            raise_from(ConnError("handshake"), ex)
 
         conn.wsuri = wsuri
         conn._ws = ws
@@ -110,7 +110,7 @@ class Conn:
         conn._rx_wg.wait()
         err = conn._down_err  # no need to lock after shutdown/_rx_wg.wait()
         if not isinstance(err, ConnClosedError):
-            raise ConnError("close") from err
+            raise_from(ConnError("close"), err)
 
     # _shutdown brings the connection down due to err.
     # only the first call has effect.
@@ -205,7 +205,7 @@ class Conn:
         _, ok = _rx
         if not ok:
             # NOTE no need to lock - rxq is closed after ._down_err is set
-            raise ConnError("recv") from conn._down_err
+            raise_from(ConnError("recv"), conn._down_err)
 
         rx, rx_raw = _
         return (rx, rx_raw)
@@ -231,7 +231,7 @@ class Conn:
             # FIXME handle ctx cancel  (but it won't stuck forever due to ._ws own timeout)
             conn._ws.send(jmsg)
         except Exception as ex:
-            raise ConnError("send") from ex
+            raise_from(ConnError("send"), ex)
         return rxq
 
 
@@ -250,3 +250,11 @@ class Conn:
     @property
     def srv_version(conn):
         return conn.srv_ready_msg['version']
+
+
+
+# py2 compat
+def raise_from(exc, cause):
+    exc.__cause__ = cause
+    exc.__context__ = None
+    raise exc

amari/kpi.py

@@ -26,7 +26,7 @@ The KPIs themselves can be computed from Measurements via package xlte.kpi .
 from __future__ import print_function, division, absolute_import
 
 from xlte import kpi
-from xlte.amari import xlog
+from xlte.amari import xlog, raise_from
 from golang import func
 
 
@@ -348,7 +348,7 @@ def _stats_check(stats):
         stats.get1("counters", dict).get1("messages", dict)
         stats.get1("cells", dict).get1(cellname, dict).get1("counters", dict).get1("messages", dict)
     except Exception as e:
-        raise LogError(stats.timestamp, "stats: %s" % e)  from None
+        raise_from(LogError(stats.timestamp, "stats: %s" % e), None)
     return
 
 # _stats_cc returns specified cumulative counter from stats result.

amari/xlog.py

@@ -66,7 +66,7 @@ LOS_window = 1000
 
 
 from xlte import amari
-from xlte.amari import drb
+from xlte.amari import drb, raise_from
 
 import json
 import traceback
@@ -644,7 +644,7 @@ def _jread1(xr): # -> xdict|None
     try:
         l = xr._r.readline()
     except Exception as e:
-        raise xr._err("read") from e
+        raise_from(xr._err("read"), e)
 
     if len(l) == 0:
         return None # EOF
@@ -652,7 +652,7 @@ def _jread1(xr): # -> xdict|None
     try:
         d = json.loads(l)
     except Exception as e:
-        raise xr._err("invalid json: %s" % e) from None
+        raise_from(xr._err("invalid json: %s" % e), None)
 
     if not isinstance(d, dict):
         raise xr._err("got %s instead of dict" % type(d))