diff --git a/numjuggler/likefunc.py b/numjuggler/likefunc.py
index 33b9174..7943dcf 100644
--- a/numjuggler/likefunc.py
+++ b/numjuggler/likefunc.py
@@ -7,7 +7,8 @@
 
 from __future__ import print_function
 from collections import OrderedDict
-import sys
+
+from numjuggler.utils.io import resolve_fname_or_stream
 
 
 def trivial(x):
@@ -68,29 +69,19 @@ def __init__(self, log=False):
         self.doc = ""
         return
 
-    def __call1(self, x):
-        res = self.get_value(x)
-        return res
-
-    def __call2(self, x):
+    def __call__(self, x):
         res = self.get_value(x)
-        self.ld[x] = res
+        if self.log:
+            self.ld[x] = res
         return res
 
-    def __call__(self, x):
-        return self.__call1(x)
-
     @property
     def log(self):
         return self.__lf
 
     @log.setter
-    def log(self, v):
-        if v:
-            self.__call__ = self.__call2
-        else:
-            self.__call__ = self.__call1
-        self.__ld = v
+    def log(self, _log):
+        self.__lf = _log
 
     def __str__(self):
         res = []
@@ -101,25 +92,12 @@ def __str__(self):
         res.append('other -> {}'.format(self.default._mydoc))
         return '\n'.join(res)
 
-    def _get_log_file(self, fname):
-        if self.log is None:
+    def write_log_as_map(self, t, fname_or_stream=None):
+        if not self.log:
             raise ValueError("Cannon write log for unlogged mapping.")
-
-        # Define where to print
-        if fname is None:
-            fout = sys.stdout
-        else:
-            try:
-                fout = open(fname, 'w')
-            except:
-                print("Cannot open {}. Print to stdout".format(repr(fname)))
-                fout = sys.stdout
-        return fout
-
-    def write_log_as_map(self, t, fname=None):
-        fout = self._get_log_file(fname)
-        for nold, nnew in self.ld.items():
-            print('{} {}: {}'.format(t, nnew, nold), file=fout)
+        with resolve_fname_or_stream(fname_or_stream, "w") as fout:
+            for nold, nnew in self.ld.items():
+                print('{} {}: {}'.format(t, nnew, nold), file=fout)
 
 
 class LikeFunction(LikeFunctionBase):
@@ -261,7 +239,7 @@ def read_map_file(fname, log=False):
     # Dictionary type -> LikeFunction
     maps = {}
 
-    with open(fname, 'r') as mapfile:
+    with resolve_fname_or_stream(fname, 'r') as mapfile:
         for l in mapfile:
             t, ranges, f = _parse_map_line(l)
             if t is None:
diff --git a/numjuggler/utils/io.py b/numjuggler/utils/io.py
index 3e99910..20e6d13 100644
--- a/numjuggler/utils/io.py
+++ b/numjuggler/utils/io.py
@@ -1,4 +1,6 @@
 import os
+import sys
+
 from .resource import Path
 from contextlib import contextmanager
 
@@ -11,3 +13,18 @@ def cd_temporarily(cd_to):
         yield
     finally:
         os.chdir(cur_dir)
+
+
+@contextmanager
+def resolve_fname_or_stream(fname_or_stream, mode="r"):
+    is_input = mode == 'r'
+    if fname_or_stream is None:
+        if is_input:
+            yield sys.stdin
+        else:
+            yield sys.stdout
+    elif is_input and hasattr(fname_or_stream, "read") or not is_input and hasattr(fname_or_stream, "write"):
+        yield fname_or_stream
+    else:
+        with open(fname_or_stream, mode=mode) as fid:
+            yield fid
diff --git a/tests/test_likefunc.py b/tests/test_likefunc.py
new file mode 100644
index 0000000..a5a0a66
--- /dev/null
+++ b/tests/test_likefunc.py
@@ -0,0 +1,54 @@
+import pytest
+from numjuggler.utils.io import cd_temporarily
+from six import StringIO
+import numjuggler.likefunc as lf
+
+
+
+@pytest.mark.parametrize("data, log, present, absent, expected_present_value, expected_absent_value, expected_text", [
+    (
+        """
+            c 1: 12
+            c 2: 14
+        """,
+        False,
+        1, 3,
+        12, 3,
+        "",
+    ),
+    (
+        """
+            c 1: 12
+            c 2: 14
+        """,
+        True,
+        1, 3,
+        12, 3,
+        "cel 12: 1\ncel 3: 3\n",
+    ),
+])
+def test_LikeFunction(
+    data,
+    log,
+    present,
+    absent,
+    expected_present_value,
+    expected_absent_value,
+    expected_text
+):
+    input = StringIO(data)
+    maps = lf.read_map_file(input, log)
+    actual = StringIO()
+    for k in maps:
+        like_function = maps[k]
+        assert like_function.log == log
+        present_value = like_function(present)
+        assert present_value == expected_present_value
+        absent_value = like_function(absent)
+        assert absent_value == expected_absent_value
+        if log:
+            like_function.write_log_as_map(k, actual)
+        else:
+            with pytest.raises(ValueError):
+                like_function.write_log_as_map(k, actual)
+    assert actual.getvalue() == expected_text