diff --git a/pysnooper/tracer.py b/pysnooper/tracer.py
index 0914820..5929c87 100644
--- a/pysnooper/tracer.py
+++ b/pysnooper/tracer.py
@@ -99,6 +99,34 @@ def get_source_from_frame(frame):
     return source
 
 
+def get_write_and_truncate_functions(output):
+    truncate = None
+    if output is None:
+        def write(s):
+            stderr = sys.stderr
+            try:
+                stderr.write(s)
+            except UnicodeEncodeError:
+                # God damn Python 2
+                stderr.write(utils.shitcode(s))
+    elif isinstance(output, (pycompat.PathLike, str)):
+        def write(s):
+            with open(six.text_type(output), 'a') as output_file:
+                output_file.write(s)
+
+        def truncate():
+            with open(six.text_type(output), 'w'):
+                pass
+    elif callable(output):
+        write = output
+    else:
+        assert isinstance(output, utils.WritableStream)
+
+        def write(s):
+            output.write(s)
+    return write, truncate
+
+
 class Tracer:
     def __init__(
             self,
@@ -142,38 +170,12 @@ class Tracer:
             @pysnooper.snoop(prefix='ZZZ ')
 
         '''
-        self.truncate = None
-        if output is None:
-            def write(s):
-                stderr = sys.stderr
-                try:
-                    stderr.write(s)
-                except UnicodeEncodeError:
-                    # God damn Python 2
-                    stderr.write(utils.shitcode(s))
-        elif isinstance(output, (pycompat.PathLike, str)):
-            def write(s):
-                with open(six.text_type(output), 'a') as output_file:
-                    output_file.write(s)
-
-            def truncate():
-                with open(six.text_type(output), 'w'):
-                    pass
-
-            self.truncate = truncate
-        elif callable(output):
-            write = output
-        else:
-            assert isinstance(output, utils.WritableStream)
-
-            def write(s):
-                output.write(s)
+        self._write, self.truncate = get_write_and_truncate_functions(output)
 
         if self.truncate is None and overwrite:
             raise Exception("`overwrite=True` can only be used when writing "
                             "content to file.")
             
-        self._write = write
         self.watch = [
             v if isinstance(v, BaseVariable) else CommonVariable(v)
             for v in utils.ensure_tuple(watch)