diff --git a/python-bin/python3 b/python-bin/python3
index a29fdc63cf..111ec2d83a 100755
--- a/python-bin/python3
+++ b/python-bin/python3
@@ -2,6 +2,14 @@
 
 DEPOT_TOOLS=$(dirname "$0")/..
 
+if [[ ! -f "$DEPOT_TOOLS/python3_bin_reldir.txt" ]]; then
+  cat >&2 <<EOF
+python3_bin_reldir.txt not found. need to initialize depot_tools by
+running gclient, update_depot_tools or ensure_bootstrap.
+EOF
+  exit 1
+fi
+
 if [ "$OSTYPE" = "msys" ]
 then
   PYTHON3_BIN_DIR="$DEPOT_TOOLS/$(sed -e 's-\\-/-g' $DEPOT_TOOLS/python3_bin_reldir.txt)"
diff --git a/python-bin/python3.bat b/python-bin/python3.bat
index 6c6f42a2fd..6cabdf2e88 100644
--- a/python-bin/python3.bat
+++ b/python-bin/python3.bat
@@ -6,6 +6,13 @@
 setlocal
 
 for %%d in (%~dp0..) do set PARENT_DIR=%%~fd
+IF NOT EXIST "%PARENT_DIR%\python3_bin_reldir.txt" (
+  echo python3_bin_reldir.txt not found. need to initialize depot_tools by >&2
+  echo running gclient or update_depot_tools >&2
+  exit /b 1
+)
+
+:Initialized
 for /f %%i in (%PARENT_DIR%\python3_bin_reldir.txt) do set PYTHON_BIN_ABSDIR=%PARENT_DIR%\%%i
 set PATH=%PYTHON_BIN_ABSDIR%;%PYTHON_BIN_ABSDIR%\Scripts;%PATH%
-"%PYTHON_BIN_ABSDIR%\python3.exe" %*
\ No newline at end of file
+"%PYTHON_BIN_ABSDIR%\python3.exe" %*