Fix ExclusiveLockFile; better doctest for AtomicCreateIfMissing

ExclusiveLockFile unconditionally closed the file on object deletion,
which threw an exception if you don't have an open file.

csppfetch/exclusivelockfile.py

@@ -111,7 +111,8 @@ class ExclusiveLockFile:
         self.unlock()
 
     def __del__(self):
-        self.file.close()
+        if self.file is not None:
+            self.file.close()
 
 def samefile(f1, f2):
     """ Do do file objects correspond to the same file on disk?
@@ -193,7 +194,6 @@ def file_matches_filename(file, filename):
 
 class AtomicCreateIfMissing:
     """ Context manager to atomically create a file
-
     >>> from exclusivelockfile import AtomicCreateIfMissing
     >>> from tempfile import mkdtemp
     >>> tmpdir = mkdtemp()
@@ -201,18 +201,23 @@ class AtomicCreateIfMissing:
     >>> #
     >>> with AtomicCreateIfMissing(filename) as f:
     ...     if f is None:
-    ...         raise Exception("This won't happen, as filename doens't exist yet")
-    """
-    """
-    ...     f.write("this is the body of a test file\n")
-    ...     with open(filename) as input:
-    ...         print(input.read()) # prints nothing; file is still empty
+    ...         raise Exception("This won't happen, as filename doesn't exist yet")
+    ...     _ = f.write("this is the body of the test file")
+    ...     try:
+    ...         open(filename)
+    ...         raise Exception("This won't happen; file doesn't exist yet")
+    ...     except FileNotFoundError:
+    ...         pass
+    >>> # This file now exists
     >>> with open(filename) as f: print(f.read())
-    this is the body of a test file
+    this is the body of the test file
+    >>> # Retrying will fail, returning None as the file
     >>> with AtomicCreateIfMissing(filename) as f:
-    ...     if f is None:
-    ...         return
-    ...     raise Exception("This won't happen, as the file already exists and AtomicCreateIfMissing returned None above")
+    ...     if f is not None:
+    ...         raise Exception("This won't happen, as the file already exists and AtomicCreateIfMissing returned None above")
+    >>> import os
+    >>> os.unlink(filename)
+    >>> os.rmdir(tmpdir)
     """
 
     def __init__(self, filename, partial_suffix=".part"):