From b5a94d8e6c49201c8c79a7c52b8466e020e6d6b8 Mon Sep 17 00:00:00 2001
From: Bruno Oliveira <nicoddemus@gmail.com>
Date: Fri, 25 May 2018 17:24:23 -0300
Subject: [PATCH] Write rewritten code using file.write(marshal.dumps())

This works around the fact that some libraries might monkey patch
the file object, so the previous approach of marshal.dump(co, file)
breaks because file is not a built-in file object anymore.

Fix #3503
---
 _pytest/assertion/rewrite.py | 5 +----
 changelog/3503.bugfix.rst    | 1 +
 2 files changed, 2 insertions(+), 4 deletions(-)
 create mode 100644 changelog/3503.bugfix.rst

diff --git a/_pytest/assertion/rewrite.py b/_pytest/assertion/rewrite.py
index 0499a792f..c72f4c981 100644
--- a/_pytest/assertion/rewrite.py
+++ b/_pytest/assertion/rewrite.py
@@ -265,10 +265,7 @@ def _write_pyc(state, co, source_stat, pyc):
             mtime = int(source_stat.mtime)
             size = source_stat.size & 0xFFFFFFFF
             fp.write(struct.pack("<ll", mtime, size))
-            if six.PY2:
-                marshal.dump(co, fp.file)
-            else:
-                marshal.dump(co, fp)
+            fp.write(marshal.dumps(co))
     except EnvironmentError as e:
         state.trace("error writing pyc file at %s: errno=%s" % (pyc, e.errno))
         # we ignore any failure to write the cache file
diff --git a/changelog/3503.bugfix.rst b/changelog/3503.bugfix.rst
new file mode 100644
index 000000000..e3ffdfaaf
--- /dev/null
+++ b/changelog/3503.bugfix.rst
@@ -0,0 +1 @@
+Fix assertion rewriter compatibility with libraries that monkey patch ``file`` objects.