diff --git a/jupytext/jupytext.py b/jupytext/jupytext.py
index 08d951cd6..0ed7ff87c 100644
--- a/jupytext/jupytext.py
+++ b/jupytext/jupytext.py
@@ -191,7 +191,7 @@ def writes(self, nb, metadata=None, **kwargs):
 
 def reads(text, fmt, as_version=4, **kwargs):
     """Read a notebook from a string"""
-    fmt = copy(fmt)
+    fmt = copy(fmt) if fmt else divine_format(text)
     fmt = long_form_one_format(fmt)
     ext = fmt['extension']
 
@@ -224,21 +224,21 @@ def read(fp, as_version=4, fmt=None, **kwargs):
     """Read a notebook from a file"""
     if fp == '-':
         text = sys.stdin.read()
-        fmt = fmt or divine_format(text)
         return reads(text, fmt)
 
-    if isinstance(fp, (py3compat.unicode_type, bytes)):
-        _, ext = os.path.splitext(fp)
+    if not hasattr(fp, 'read'):
+        _, ext = os.path.splitext(str(fp))
         fmt = copy(fmt or {})
         fmt.update({'extension': ext})
         with io.open(fp, encoding='utf-8') as stream:
             return read(stream, as_version=as_version, fmt=fmt, **kwargs)
 
-    fmt = long_form_one_format(fmt)
-    if fmt['extension'] == '.ipynb':
-        notebook = nbformat.read(fp, as_version, **kwargs)
-        rearrange_jupytext_metadata(notebook.metadata)
-        return notebook
+    if fmt is not None:
+        fmt = long_form_one_format(fmt)
+        if fmt['extension'] == '.ipynb':
+            notebook = nbformat.read(fp, as_version, **kwargs)
+            rearrange_jupytext_metadata(notebook.metadata)
+            return notebook
 
     return reads(fp.read(), fmt, **kwargs)
 
@@ -283,8 +283,9 @@ def write(nb, fp, version=nbformat.NO_CONVERT, fmt=None, **kwargs):
         write(nb, sys.stdout, version=version, fmt=fmt, **kwargs)
         return
 
-    if isinstance(fp, (py3compat.unicode_type, bytes)):
+    if not hasattr(fp, 'write'):
         fmt = copy(fmt or {})
+        fp = str(fp)
         _, ext = os.path.splitext(fp)
         fmt = long_form_one_format(fmt, update={'extension': ext})
         create_prefix_dir(fp, fmt)
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 4955baafc..c1b8f940d 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -5,3 +5,6 @@ codecov
 notebook
 jupyter_client
 nbconvert
+
+# Python 2
+pathlib
\ No newline at end of file
diff --git a/tests/test_read_write_functions.py b/tests/test_read_write_functions.py
new file mode 100644
index 000000000..06ff8f134
--- /dev/null
+++ b/tests/test_read_write_functions.py
@@ -0,0 +1,78 @@
+from io import StringIO
+from pathlib import Path
+import nbformat
+from nbformat.v4.nbbase import new_notebook, new_markdown_cell
+import jupytext
+from jupytext.compare import compare
+
+
+def test_simple_hook(tmpdir):
+    nb_file = str(tmpdir.join('notebook.ipynb'))
+    md_file = str(tmpdir.join('notebook.md'))
+    nbformat.write(new_notebook(cells=[new_markdown_cell('Some text')]), nb_file)
+
+    nb = jupytext.read(nb_file)
+    jupytext.write(nb, md_file)
+
+    with open(md_file) as fp:
+        text = fp.read()
+
+    assert 'Some text' in text.splitlines()
+
+
+def test_simple_hook_with_explicit_format(tmpdir):
+    nb_file = str(tmpdir.join('notebook.ipynb'))
+    py_file = str(tmpdir.join('notebook.py'))
+    nbformat.write(new_notebook(cells=[new_markdown_cell('Some text')]), nb_file)
+
+    nb = jupytext.read(nb_file)
+    jupytext.write(nb, py_file, fmt='py:percent')
+
+    with open(py_file) as fp:
+        text = fp.read()
+
+    assert '# %% [markdown]' in text.splitlines()
+    assert '# Some text' in text.splitlines()
+
+
+def test_no_error_on_path_object(tmpdir):
+    nb_file = Path(str(tmpdir.join('notebook.ipynb')))
+    md_file = nb_file.with_suffix('.md')
+    nbformat.write(new_notebook(cells=[new_markdown_cell('Some text')]), str(nb_file))
+
+    nb = jupytext.read(nb_file)
+    jupytext.write(nb, md_file)
+
+
+def test_read_ipynb_from_stream():
+    def stream():
+        return StringIO(u"""{
+ "cells": [
+  {
+   "cell_type": "code",
+   "metadata": {},
+   "source": [
+    "1 + 1"
+   ]
+  }
+ ],
+ "metadata": {},
+ "nbformat": 4,
+ "nbformat_minor": 4
+}
+""")
+
+    nb = jupytext.read(stream())
+    nb2 = jupytext.read(stream(), fmt='ipynb')
+    compare(nb, nb2)
+
+
+def test_read_py_percent_from_stream():
+    def stream():
+        return StringIO(u"""# %%
+1 + 1
+""")
+
+    nb = jupytext.read(stream())
+    nb2 = jupytext.read(stream(), fmt='py:percent')
+    compare(nb, nb2)