diff --git a/inline-python.cabal b/inline-python.cabal
index 970e0c7..9ed71fe 100644
--- a/inline-python.cabal
+++ b/inline-python.cabal
@@ -66,10 +66,10 @@ Library
   pkgconfig-depends: python3-embed
   --
   Exposed-modules:
+    Python.Inline
     Python.Inline.Literal
     Python.Inline.QQ
-    Python.Inline
-    Python.Types
+    Python.Inline.Types
   Other-modules:
     Python.Internal.CAPI
     Python.Internal.Eval
diff --git a/src/Python/Inline.hs b/src/Python/Inline.hs
index 4e34726..bb22fdf 100644
--- a/src/Python/Inline.hs
+++ b/src/Python/Inline.hs
@@ -55,8 +55,8 @@ module Python.Inline
   , FromPy
   ) where
 
-import Python.Types
 import Python.Inline.Literal
+import Python.Internal.Types
 import Python.Internal.Eval
 
 
diff --git a/src/Python/Inline/Literal.hs b/src/Python/Inline/Literal.hs
index 201b6bc..76fed0e 100644
--- a/src/Python/Inline/Literal.hs
+++ b/src/Python/Inline/Literal.hs
@@ -28,7 +28,6 @@ import GHC.Float                 (float2Double, double2Float)
 import Language.C.Inline         qualified as C
 import Language.C.Inline.Unsafe  qualified as CU
 
-import Python.Types
 import Python.Internal.Types
 import Python.Internal.Eval
 import Python.Internal.CAPI
@@ -104,7 +103,7 @@ fromPy' py = unsafeWithPyObject py basicFromPy
 -- | Convert haskell value to a python object.
 toPy :: ToPy a => a -> Py PyObject
 toPy a = basicToPy a >>= \case
-  NULL -> throwM =<< convertPy2Haskell
+  NULL -> mustThrowPyError
   p    -> newPyObject p
 
 
diff --git a/src/Python/Types.hs b/src/Python/Inline/Types.hs
similarity index 88%
rename from src/Python/Types.hs
rename to src/Python/Inline/Types.hs
index f6cd4e2..fe45b45 100644
--- a/src/Python/Types.hs
+++ b/src/Python/Inline/Types.hs
@@ -1,6 +1,6 @@
 -- |
 -- Data types and utilities.
-module Python.Types
+module Python.Inline.Types
   ( -- * @Py@ monad
     Py
   , runPy
diff --git a/src/Python/Internal/Eval.hs b/src/Python/Internal/Eval.hs
index b3d20ed..1c79d93 100644
--- a/src/Python/Internal/Eval.hs
+++ b/src/Python/Internal/Eval.hs
@@ -40,6 +40,7 @@ import Control.Monad
 import Control.Monad.Catch
 import Control.Monad.IO.Class
 import Control.Monad.Trans.Cont
+import Data.Maybe
 import Foreign.Concurrent        qualified as GHC
 import Foreign.Ptr
 import Foreign.ForeignPtr
@@ -53,6 +54,7 @@ import System.IO.Unsafe
 import Language.C.Inline          qualified as C
 import Language.C.Inline.Unsafe   qualified as CU
 
+import Python.Internal.CAPI
 import Python.Internal.Types
 import Python.Internal.Util
 import Python.Internal.Program
@@ -231,8 +233,8 @@ callbackEnsurePyLock action = do
 
 acquireLock :: ThreadId -> STM ()
 acquireLock tid = readTVar globalPyLock >>= \case
-  LockUninialized -> error "Python is not started"
-  LockFinalized   -> error "Python is already stopped"
+  LockUninialized -> throwSTM PythonNotInitialized
+  LockFinalized   -> throwSTM PythonIsFinalized
   LockedByGC      -> retry
   LockUnlocked    -> writeTVar globalPyLock $ Locked tid []
   Locked t xs
@@ -241,20 +243,20 @@ acquireLock tid = readTVar globalPyLock >>= \case
 
 grabLock :: ThreadId -> STM ()
 grabLock tid = readTVar globalPyLock >>= \case
-  LockUninialized -> error "Python is not started"
-  LockFinalized   -> error "Python is already stopped"
+  LockUninialized -> throwSTM PythonNotInitialized
+  LockFinalized   -> throwSTM PythonIsFinalized
   LockedByGC      -> retry
   LockUnlocked    -> writeTVar globalPyLock $ Locked tid []
   Locked t xs     -> writeTVar globalPyLock $ Locked tid (t : xs)
 
 releaseLock :: ThreadId -> STM ()
 releaseLock tid = readTVar globalPyLock >>= \case
-  LockUninialized -> error "Python is not started"
-  LockFinalized   -> error "Python is already stopped"
-  LockUnlocked    -> error "INTERNAL ERROR releasing unlocked"
-  LockedByGC      -> error "INTERNAL ERROR lock held by GC"
+  LockUninialized -> throwSTM PythonNotInitialized
+  LockFinalized   -> throwSTM PythonIsFinalized
+  LockUnlocked    -> throwSTM $ PyInternalError "releaseLock: releasing LockUnlocked"
+  LockedByGC      -> throwSTM $ PyInternalError "releaseLock: releasing LockedByGC"
   Locked t xs
-    | t /= tid  -> error "INTERNAL ERROR releasing wrong lock"
+    | t /= tid  -> throwSTM $ PyInternalError "releaseLock: releasing  wrong lock"
     | otherwise -> writeTVar globalPyLock $! case xs of
         []    -> LockUnlocked
         t':ts -> Locked t' ts
@@ -290,8 +292,8 @@ doInializePython = do
   -- First we need to grab global python lock on haskell side
   join $ atomically $ do
     readTVar globalPyState >>= \case
-      Finalized        -> error "Python was already finalized"
-      InitFailed       -> error "Python was unable to initialize"
+      Finalized        -> throwSTM PythonNotInitialized
+      InitFailed       -> throwSTM PythonIsFinalized
       InInitialization -> retry
       InFinalization   -> retry
       Running1{}       -> pure $ pure ()
@@ -401,8 +403,8 @@ doInializePythonIO = do
 
 doFinalizePython :: IO ()
 doFinalizePython = join $ atomically $ readTVar globalPyState >>= \case
-  NotInitialized   -> error "Python is not initialized"
-  InitFailed       -> error "Python failed to initialize"
+  NotInitialized   -> throwSTM PythonNotInitialized
+  InitFailed       -> throwSTM PythonIsFinalized
   Finalized        -> pure $ pure ()
   InInitialization -> retry
   InFinalization   -> retry
@@ -419,8 +421,8 @@ doFinalizePython = join $ atomically $ readTVar globalPyState >>= \case
     takeMVar resp
   where
     checkLock action = readTVar globalPyLock >>= \case
-      LockUninialized -> error "Internal error: Lock not initialized"
-      LockFinalized   -> error "Internal error: Lock is already finalized"
+      LockUninialized -> throwSTM $ PyInternalError "doFinalizePython LockUninialized"
+      LockFinalized   -> throwSTM $ PyInternalError "doFinalizePython LockFinalized"
       Locked{}        -> retry
       LockedByGC      -> retry
       LockUnlocked    -> do
@@ -459,12 +461,12 @@ runPyInMain :: Py a -> IO a
 runPyInMain py
   -- Multithreaded RTS
   | rtsSupportsBoundThreads = join $ atomically $ readTVar globalPyState >>= \case
-      NotInitialized   -> error "Python is not initialized"
-      InitFailed       -> error "Python failed to initialize"
-      Finalized        -> error "Python is already finalized"
+      NotInitialized   -> throwSTM PythonNotInitialized
+      InitFailed       -> throwSTM PyInitializationFailed
+      Finalized        -> throwSTM PythonIsFinalized
       InInitialization -> retry
       InFinalization   -> retry
-      Running1         -> error "INTERNAL ERROR"
+      Running1         -> throwSTM $ PyInternalError "runPyInMain: Running1"
       RunningN _ eval tid_main _ -> do
         acquireLock tid_main
         pure
@@ -572,10 +574,9 @@ convertHaskell2Py err = Py $ do
 
 -- | Convert python exception to haskell exception. Should only be
 --   called if there's unhandled python exception. Clears exception.
-convertPy2Haskell :: Py PyError
+convertPy2Haskell :: Py PyException
 convertPy2Haskell = runProgram $ do
   p_errors <- withPyAllocaArray @(Ptr PyObject) 3
-  p_len    <- withPyAlloca      @CLong
   -- Fetch error indicator
   (p_type, p_value) <- progIO $ do
     [CU.block| void {
@@ -587,40 +588,24 @@ convertPy2Haskell = runProgram $ do
     -- Traceback is not used ATM
     pure (p_type,p_value)
   -- Convert exception type and value to strings.
-  let pythonStr p = do
-        p_str <- progIO [CU.block| PyObject* {
-          PyObject *s = PyObject_Str($(PyObject *p));
-          if( PyErr_Occurred() ) {
-              PyErr_Clear();
-          }
-          return s;
-          } |]
-        case p_str of
-          NULL -> abort UncovertablePyError
-          _    -> pure p_str
-  s_type  <- takeOwnership =<< pythonStr p_type
-  s_value <- takeOwnership =<< pythonStr p_value
-  -- Convert to haskell strings
-  let toString p = do
-        c_str <- [CU.block| const char* {
-          const char* s = PyUnicode_AsUTF8AndSize($(PyObject *p), $(long *p_len));
-          if( PyErr_Occurred() ) {
-              PyErr_Clear();
-          }
-          return s;
-          } |]
-        case c_str of
-          NULL -> pure ""
-          _    -> peekCString c_str
-  progIO $ PyError <$> toString s_type <*> toString s_value
-
+  progPy $ do
+    s_type  <- pyobjectStrAsHask p_type
+    s_value <- pyobjectStrAsHask p_value
+    incref p_value
+    exc     <- newPyObject p_value
+    let bad_str = "__str__ call failed"
+    pure $ PyException
+      { ty        = fromMaybe bad_str s_type
+      , str       = fromMaybe bad_str s_value
+      , exception = exc
+      }
 
 -- | Throw python error as haskell exception if it's raised.
 checkThrowPyError :: Py ()
 checkThrowPyError =
   Py [CU.exp| PyObject* { PyErr_Occurred() } |] >>= \case
     NULL -> pure ()
-    _    -> throwM =<< convertPy2Haskell
+    _    -> throwM . PyError =<< convertPy2Haskell
 
 -- | Throw python error as haskell exception if it's raised. If it's
 --   not that internal error. Another exception will be raised
@@ -628,7 +613,7 @@ mustThrowPyError :: Py a
 mustThrowPyError =
   Py [CU.exp| PyObject* { PyErr_Occurred() } |] >>= \case
     NULL -> error $ "mustThrowPyError: no python exception raised."
-    _    -> throwM =<< convertPy2Haskell
+    _    -> throwM . PyError =<< convertPy2Haskell
 
 -- | Calls mustThrowPyError if pointer is null or returns it unchanged
 throwOnNULL :: Ptr PyObject -> Py (Ptr PyObject)
diff --git a/src/Python/Internal/EvalQQ.hs b/src/Python/Internal/EvalQQ.hs
index 187ad09..98dde98 100644
--- a/src/Python/Internal/EvalQQ.hs
+++ b/src/Python/Internal/EvalQQ.hs
@@ -13,6 +13,7 @@ module Python.Internal.EvalQQ
   ) where
 
 import Control.Monad.IO.Class
+import Control.Monad.Catch
 import Data.Bits
 import Data.Char
 import Data.List                 (intercalate)
@@ -29,7 +30,6 @@ import Language.C.Inline.Unsafe   qualified as CU
 import Language.Haskell.TH.Lib    qualified as TH
 import Language.Haskell.TH.Syntax qualified as TH
 
-import Python.Types
 import Python.Internal.Types
 import Python.Internal.Program
 import Python.Internal.Eval
@@ -125,7 +125,7 @@ evaluatorPyf getSource = runProgram $ do
     pyExecExpr p_globals p_locals =<< getSource p_kwargs
     -- Look up function
     p_fun <- getFunctionObject p_locals >>= \case
-      NULL -> error "INTERNAL ERROR: _inline_python_ must be present"
+      NULL -> throwM $ PyInternalError "_inline_python_ must be present"
       p    -> pure p
     -- Call python function we just constructed
     newPyObject =<< throwOnNULL =<< basicCallKwdOnly p_fun p_kwargs
@@ -204,7 +204,7 @@ expQQ mode qq_src = do
                 ]
     case code of
       ExitSuccess   -> pure $ words stdout
-      ExitFailure{} -> error stderr
+      ExitFailure{} -> fail stderr
   let args = [ [| basicBindInDict $(TH.lift nm) $(TH.dyn (chop nm)) |]
              | nm <- antis
              ]
diff --git a/src/Python/Internal/Program.hs b/src/Python/Internal/Program.hs
index 9499a33..1152285 100644
--- a/src/Python/Internal/Program.hs
+++ b/src/Python/Internal/Program.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE QuasiQuotes     #-}
+{-# LANGUAGE TemplateHaskell #-}
 -- |
 module Python.Internal.Program
   ( Program(..)
@@ -7,6 +9,7 @@ module Python.Internal.Program
     -- * Control flow
   , abort
   , abortM
+  , abortOnNull
   , checkNull
   , finallyProg
   , onExceptionProg
@@ -17,8 +20,11 @@ module Python.Internal.Program
   , withPyCString
   , withPyCStringLen
   , withPyWCString
+    -- * Helpers
+  , pyobjectStrAsHask
   ) where
 
+import Control.Monad
 import Control.Monad.Trans.Cont
 import Control.Monad.Trans.Class
 import Control.Monad.Catch
@@ -30,10 +36,18 @@ import Foreign.C.String
 import Foreign.C.Types
 import Foreign.Storable
 
+import Language.C.Inline          qualified as C
+import Language.C.Inline.Unsafe   qualified as CU
+
 import Python.Internal.Types
 import Python.Internal.Util
 import Python.Internal.CAPI
 
+----------------------------------------------------------------
+C.context (C.baseCtx <> pyCtx)
+C.include "<inline-python.h>"
+----------------------------------------------------------------
+
 
 -- | This monad wraps 'Py' into 'ContT' in order get early exit,
 --   applying @finally@ while avoiding building huge ladders.
@@ -58,11 +72,15 @@ abort r = Program $ ContT $ \_ -> pure r
 abortM :: Py r -> Program r a
 abortM m = Program $ ContT $ \_ -> m
 
+-- | Perform early exit if pointer is null
+abortOnNull :: r -> Py (Ptr a) -> Program r (Ptr a)
+abortOnNull r action = Program $ ContT $ \cnt -> action >>= \case
+  NULL -> pure r
+  p    -> cnt p
+
 -- | If result of computation is NULL return NULL immediately.
 checkNull :: Py (Ptr a) -> Program (Ptr a) (Ptr a)
-checkNull action = Program $ ContT $ \cnt -> action >>= \case
-  NULL -> pure nullPtr
-  p    -> cnt p
+checkNull = abortOnNull nullPtr
 
 -- | Evaluate finalizer even if exception is thrown.
 finallyProg
@@ -99,3 +117,28 @@ withPyWCString = coerce (withWCString @r)
 
 withPyCStringLen :: forall r. String -> Program r CStringLen
 withPyCStringLen = coerce (withCStringLen @r)
+
+
+----------------------------------------------------------------
+-- More complicated helpers
+----------------------------------------------------------------
+
+-- | Call @__str__@ method of object and return haskell
+--   string. Returns Nothing if exception was raisede
+pyobjectStrAsHask :: Ptr PyObject -> Py (Maybe String)
+pyobjectStrAsHask p_obj = runProgram $ do
+  p_str <- takeOwnership <=< abortOnNull Nothing $ Py [CU.block| PyObject* {
+    PyObject *s = PyObject_Str($(PyObject *p_obj));
+    if( PyErr_Occurred() ) {
+        PyErr_Clear();
+    }
+    return s;
+    } |]
+  c_str <- abortOnNull Nothing $ Py [CU.block| const char* {
+    const char* s = PyUnicode_AsUTF8($(PyObject *p_str));
+    if( PyErr_Occurred() ) {
+        PyErr_Clear();
+    }
+    return s;
+    } |]
+  progIO $ Just <$> peekCString c_str
diff --git a/src/Python/Internal/Types.hs b/src/Python/Internal/Types.hs
index 553edf1..1c137b0 100644
--- a/src/Python/Internal/Types.hs
+++ b/src/Python/Internal/Types.hs
@@ -11,6 +11,8 @@ module Python.Internal.Types
   , unsafeWithPyObject
   , PyThreadState
   , PyError(..)
+  , PyException(..)
+  , PyInternalError(..)
   , Py(..)
   , pyIO
     -- * inline-C
@@ -46,18 +48,15 @@ data PyThreadState
 -- | Some python object. Since almost everything in python is mutable
 --   it could only be accessed only in IO monad.
 newtype PyObject = PyObject (ForeignPtr PyObject)
+  deriving stock Show
 
 unsafeWithPyObject :: forall a. PyObject -> (Ptr PyObject -> Py a) -> Py a
 unsafeWithPyObject = coerce (unsafeWithForeignPtr @PyObject @a)
 
 -- | Python exception converted to haskell.
 data PyError
-  = PyError String String
+  = PyError !PyException
     -- ^ Python exception. Contains exception type and message as strings.
-  | UncovertablePyError
-    -- ^ Python exception that could not be converted to haskell for
-    --   some reason. Its appearance means that something went
-    --   seriously wrong.
   | BadPyType
     -- ^ It's not possible to convert given python value to a haskell
     --   value
@@ -67,10 +66,26 @@ data PyError
     --   result in this exception.
   | PyInitializationFailed
     -- ^ Initialization of python interpreter failed
-  deriving stock (Show)
-
-instance Exception PyError
-
+  | PythonNotInitialized
+    -- ^ Python interpreter is not initialized
+  | PythonIsFinalized
+    -- ^ Python interpreter is not initialized    
+  deriving stock    (Show)
+  deriving anyclass (Exception)
+
+-- | Python exception converted to haskell value
+data PyException = PyException
+  { ty        :: !String   -- ^ Exception type as a string
+  , str       :: !String   -- ^ String representation of an exception
+  , exception :: !PyObject -- ^ Exception object
+  }
+  deriving stock Show
+
+-- | Internal error. If this exception is thrown it means there's bug
+--   in a library.
+data PyInternalError = PyInternalError String
+  deriving stock    (Show)
+  deriving anyclass (Exception)
 
 -- | Monad for code which is interacts with python interpreter. Only
 --   one haskell thread can interact with python interpreter at a
diff --git a/test/TST/FromPy.hs b/test/TST/FromPy.hs
index 17bfed8..8329149 100644
--- a/test/TST/FromPy.hs
+++ b/test/TST/FromPy.hs
@@ -7,7 +7,7 @@ import Test.Tasty
 import Test.Tasty.HUnit
 import Python.Inline
 import Python.Inline.QQ
-import Python.Types
+import Python.Inline.Types
 
 tests :: TestTree
 tests = testGroup "FromPy"
diff --git a/test/TST/Util.hs b/test/TST/Util.hs
index 8843f2b..8295cd4 100644
--- a/test/TST/Util.hs
+++ b/test/TST/Util.hs
@@ -6,7 +6,7 @@ import Control.Monad.Catch
 import Test.Tasty.HUnit
 
 import Python.Inline
-import Python.Types
+import Python.Inline.Types
 
 throwsPy :: Py () -> Py ()
 throwsPy io = (io >> liftIO (assertFailure "Evaluation should raise python exception"))