bpo-39683: 2to3 fix_exitfunc suggests duplicated import of atexit module

2to3 fix for sys.exitfunc adds multiple import when sys.exitfunc
is present multiple times.

This patch adds a check for already existing 'import atexit' and
do not add multiple imports.

Lib/lib2to3/fixes/fix_exitfunc.py

@@ -5,7 +5,7 @@ Convert use of sys.exitfunc to use the atexit module.
 # Author: Benjamin Peterson
 
 from lib2to3 import pytree, fixer_base
-from lib2to3.fixer_util import Name, Attr, Call, Comma, Newline, syms
+from lib2to3.fixer_util import Name, Attr, Call, Comma, Newline, syms, does_tree_import
 
 
 class FixExitfunc(fixer_base.BaseFix):
@@ -55,6 +55,10 @@ class FixExitfunc(fixer_base.BaseFix):
                              "import at the top of your file.")
             return
 
+        # Do not add import if already present (multiple sys.atexit's)
+        if does_tree_import(None, "atexit", self.sys_import.parent):
+            return
+
         # Now add an atexit import after the sys import.
         names = self.sys_import.children[1]
         if names.type == syms.dotted_as_names:
@@ -63,7 +67,6 @@ class FixExitfunc(fixer_base.BaseFix):
         else:
             containing_stmt = self.sys_import.parent
             position = containing_stmt.children.index(self.sys_import)
-            stmt_container = containing_stmt.parent
             new_import = pytree.Node(syms.import_name,
                               [Name("import"), Name("atexit", " ")]
                               )

Lib/lib2to3/tests/test_fixers.py

@@ -4527,6 +4527,24 @@ class Test_exitfunc(FixerTestCase):
             """
         self.check(b, a)
 
+    def test_multiple(self):
+        b = """
+            import sys
+            if 42:
+                sys.exitfunc = my_atexit
+            else:
+                sys.exitfunc = my_otheratexit
+            """
+        a = """
+            import sys
+            import atexit
+            if 42:
+                atexit.register(my_atexit)
+            else:
+                atexit.register(my_otheratexit)
+            """
+        self.check(b, a)
+
     def test_names_import(self):
         b = """
             import sys, crumbs

Misc/NEWS.d/next/Tools-Demos/2020-02-25-01-08-58.bpo-39683.-fbPik.rst

@@ -0,0 +1,2 @@
+lib2to3: Fix duplicated atexit import when fixing multiple sys.exitfunc.
+Patch by Paulo Henrique Silva.