Fix test_dagascii tests

tests/unit/test_dagascii.py

@@ -1,12 +1,11 @@
-import os
-
 from dvc import dagascii
 from dvc.env import DVC_PAGER
 
 
 def test_find_pager_uses_default_pager_when_found(mocker):
+    mocker.patch("sys.stdout.isatty", return_value=True)
+    mocker.patch("os.system", return_value=0)
     m_make_pager = mocker.patch.object(dagascii, "make_pager")
-    mocker.patch.object(os, "system", return_value=0)
 
     dagascii.find_pager()
 
@@ -14,7 +13,8 @@ def test_find_pager_uses_default_pager_when_found(mocker):
 
 
 def test_find_pager_returns_plain_pager_when_default_missing(mocker):
-    mocker.patch.object(os, "system", return_value=1)
+    mocker.patch("sys.stdout.isatty", return_value=True)
+    mocker.patch("os.system", return_value=1)
 
     pager = dagascii.find_pager()
 
@@ -24,9 +24,18 @@ def test_find_pager_returns_plain_pager_when_default_missing(mocker):
 def test_find_pager_uses_custom_pager_when_env_var_is_defined(
     mocker, monkeypatch
 ):
+    mocker.patch("sys.stdout.isatty", return_value=True)
     m_make_pager = mocker.patch.object(dagascii, "make_pager")
     monkeypatch.setenv(DVC_PAGER, dagascii.DEFAULT_PAGER)
 
     dagascii.find_pager()
 
     m_make_pager.assert_called_once_with(dagascii.DEFAULT_PAGER)
+
+
+def test_find_pager_returns_plain_pager_when_is_not_atty(mocker):
+    mocker.patch("sys.stdout.isatty", return_value=False)
+
+    pager = dagascii.find_pager()
+
+    assert pager.__name__ == "plainpager"