Automatically install pytest from CMake

Pytest is a development dependency but we can make it painless by
automating the install using CMake.

.travis.yml

@@ -39,7 +39,7 @@ matrix:
     install:
     - >
       docker exec --tty "$containerid" apt-get -y --no-install-recommends install
-      python2.7-dev python-scipy libeigen3-dev
+      python2.7-dev python-pip python-setuptools python-scipy libeigen3-dev
       cmake make g++
   - compiler: gcc-6
     services: docker
@@ -48,7 +48,7 @@ matrix:
     install:
     - >
       docker exec --tty "$containerid" apt-get -y --no-install-recommends install
-      python3.5-dev python3-scipy libeigen3-dev
+      python3.5-dev python3-pip python3-setuptools python3-scipy libeigen3-dev
       cmake make g++
   # Documentation build:
   - os: linux

tests/CMakeLists.txt

@@ -58,6 +58,31 @@ if(NOT CMAKE_LIBRARY_OUTPUT_DIRECTORY)
   endforeach()
 endif()
 
+# Make sure pytest is found or try to install it if it's not found
+macro(pybind11_execute_python)
+  execute_process(COMMAND ${PYTHON_EXECUTABLE} -m ${ARGN} OUTPUT_QUIET ERROR_QUIET
+                  RESULT_VARIABLE pybind11_execute_python_error)
+endmacro()
+
+if(NOT PYBIND11_PYTEST_FOUND)
+  pybind11_execute_python(pytest --version --noconftest)
+  if(pybind11_execute_python_error)
+    message(STATUS "Installing pytest using pip")
+    pybind11_execute_python(pip install pytest)
+    if(pybind11_execute_python_error)
+      message(STATUS "Installing pytest using pip --user (fallback)")
+      pybind11_execute_python(pip install --user pytest)
+    endif()
+
+    pybind11_execute_python(pytest --version --noconftest)
+    if(pybind11_execute_python_error)
+      message(FATAL_ERROR "Running the tests requires pytest. Please install it manually.")
+    endif()
+  endif()
+
+  set(PYBIND11_PYTEST_FOUND TRUE CACHE INTERAL "")
+endif()
+
 # A single command to compile and run the tests
-add_custom_target(pytest COMMAND ${PYTHON_EXECUTABLE} -m pytest
+add_custom_target(pytest COMMAND ${PYTHON_EXECUTABLE} -m pytest -rws
                   DEPENDS pybind11_tests WORKING_DIRECTORY ${testdir})