(core) Use fake stdout/stderr when evaluating formulas

Summary:
In the future, it may be helpful to capture some output to display in the
formula editor (e.g. the result of calling print()), but for now it's best
to redirect stdout/stderr to avoid excessive logging.

Test Plan: Tested manually.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3919

sandbox/grist/engine.py

@@ -24,6 +24,7 @@ from codebuilder import DOLLAR_REGEX
 import depend
 import docactions
 import docmodel
+from fake_std_streams import FakeStdStreams
 import gencode
 import logger
 import match_counter
@@ -944,9 +945,11 @@ class Engine(object):
         raise depend.CircularRefError("Circular Reference")
       if not col.is_formula():
         value = col.get_cell_value(int(record), restore=True)
-        result = col.method(record, table.user_table, value, self._user)
+        with FakeStdStreams():
+          result = col.method(record, table.user_table, value, self._user)
       else:
-        result = col.method(record, table.user_table)
+        with FakeStdStreams():
+          result = col.method(record, table.user_table)
       if self._cell_required_error:
         raise self._cell_required_error  # pylint: disable=raising-bad-type
       self.formula_tracer(col, record)

sandbox/grist/fake_std_streams.py

@@ -0,0 +1,18 @@
+import sys
+
+import six
+
+
+class FakeStdStreams(object):
+  """
+  Redirects stdout and stderr to StringIO.
+  """
+  def __enter__(self):
+    self._orig_stdout = sys.stdout
+    self._orig_stderr = sys.stderr
+    sys.stdout = six.StringIO()
+    sys.stderr = six.StringIO()
+
+  def __exit__(self, exc_type, exc_val, exc_tb):
+    sys.stdout = self._orig_stdout
+    sys.stderr = self._orig_stderr