OR interpreter constraints when multiple given (#678)

## Problem
Resolves #655. 

We would expect a list of constraints, such as `['CPython>=2.7,<3', 'CPython>=3.4']`, to OR the separate list entries, i.e. allow an interpreter that satisfies any of these distinct requirement strings. Instead, we always AND lists of constraints.

## Solution
Always `OR` a list of interpreter constraints. If the user wants `AND`, they may do so within the single requirement string via `,`, e.g. `'CPython>=2.7,<3'`.

pex/bin/pex.py

@@ -317,10 +317,10 @@ def configure_clp_pex_environment(parser):
     default=[],
     type='str',
     action='append',
-    help='A constraint that determines the interpreter compatibility for '
-         'this pex, using the Requirement-style format, e.g. "CPython>=3", or ">=2.7" '
-         'for requirements agnostic to interpreter class. This option can be passed multiple '
-         'times.')
+    help='Constrain the selected Python interpreter. Specify with Requirement-style syntax, '
+         'e.g. "CPython>=2.7,<3" (A CPython interpreter with version >=2.7 AND version <3) '
+         'or "PyPy" (A pypy interpreter of any version). This argument may be repeated multiple '
+         'times to OR the constraints.')
 
   group.add_option(
     '--rcfile',

pex/interpreter_constraints.py

@@ -21,21 +21,18 @@ def validate_constraints(constraints):
       die("Compatibility requirements are not formatted properly: %s" % str(e))
 
 
-def matched_interpreters(interpreters, constraints, meet_all_constraints=False):
-  """Given some filters, yield any interpreter that matches at least one of them, or all of them
-     if meet_all_constraints is set to True.
+def matched_interpreters(interpreters, constraints):
+  """Given some filters, yield any interpreter that matches at least one of them.
 
   :param interpreters: a list of PythonInterpreter objects for filtering
   :param constraints: A sequence of strings that constrain the interpreter compatibility for this
-    pex, using the Requirement-style format, e.g. ``'CPython>=3', or just ['>=2.7','<3']``
-    for requirements agnostic to interpreter class.
-  :param meet_all_constraints: whether to match against all filters.
-    Defaults to matching interpreters that match at least one filter.
+    pex. Each string uses the Requirement-style format, e.g. 'CPython>=3' or '>=2.7,<3' for
+    requirements agnostic to interpreter class. Multiple requirement strings may be combined
+    into a list to OR the constraints, such as ['CPython>=2.7,<3', 'CPython>=3.4'].
   :return interpreter: returns a generator that yields compatible interpreters
   """
-  check = all if meet_all_constraints else any
   for interpreter in interpreters:
-    if check(interpreter.identity.matches(filt) for filt in constraints):
+    if any(interpreter.identity.matches(filt) for filt in constraints):
       TRACER.log("Constraints on interpreters: %s, Matching Interpreter: %s"
                  % (constraints, interpreter.binary), V=3)
       yield interpreter

pex/pex_bootstrapper.py

@@ -85,8 +85,11 @@ def find_compatible_interpreters(pex_python_path, compatibility_constraints):
       # get all qualifying interpreters found in $PATH
       interpreters = PythonInterpreter.all()
 
-  return list(matched_interpreters(
-    interpreters, compatibility_constraints, meet_all_constraints=True))
+  return list(
+    matched_interpreters(interpreters, compatibility_constraints)
+    if compatibility_constraints
+    else interpreters
+  )
 
 
 def _select_pex_python_interpreter(target_python, compatibility_constraints):
@@ -96,7 +99,7 @@ def _select_pex_python_interpreter(target_python, compatibility_constraints):
     die('Failed to find interpreter specified by PEX_PYTHON: %s' % target)
   if compatibility_constraints:
     pi = PythonInterpreter.from_binary(target)
-    if not list(matched_interpreters([pi], compatibility_constraints, meet_all_constraints=True)):
+    if not list(matched_interpreters([pi], compatibility_constraints)):
       die('Interpreter specified by PEX_PYTHON (%s) is not compatible with specified '
           'interpreter constraints: %s' % (target, str(compatibility_constraints)))
   if not os.path.exists(target):

tests/test_integration.py

@@ -334,12 +334,12 @@ def test_interpreter_constraints_to_pex_info_py2():
     # target python 2
     pex_out_path = os.path.join(output_dir, 'pex_py2.pex')
     res = run_pex_command(['--disable-cache',
-      '--interpreter-constraint=>=2.7',
-      '--interpreter-constraint=<3',
+      '--interpreter-constraint=>=2.7,<3',
+      '--interpreter-constraint=>=3.5',
       '-o', pex_out_path])
     res.assert_success()
     pex_info = get_pex_info(pex_out_path)
-    assert set(['>=2.7', '<3']) == set(pex_info.interpreter_constraints)
+    assert {'>=2.7,<3', '>=3.5'} == set(pex_info.interpreter_constraints)
 
 
 @pytest.mark.skipif(IS_PYPY)
@@ -359,12 +359,26 @@ def test_interpreter_resolution_with_constraint_option():
   with temporary_dir() as output_dir:
     pex_out_path = os.path.join(output_dir, 'pex1.pex')
     res = run_pex_command(['--disable-cache',
-      '--interpreter-constraint=>=2.7',
-      '--interpreter-constraint=<3',
+      '--interpreter-constraint=>=2.7,<3',
       '-o', pex_out_path])
     res.assert_success()
     pex_info = get_pex_info(pex_out_path)
-    assert set(['>=2.7', '<3']) == set(pex_info.interpreter_constraints)
+    assert ['>=2.7,<3'] == pex_info.interpreter_constraints
+    assert pex_info.build_properties['version'][0] < 3
+
+
+def test_interpreter_resolution_with_multiple_constraint_options():
+  with temporary_dir() as output_dir:
+    pex_out_path = os.path.join(output_dir, 'pex1.pex')
+    res = run_pex_command(['--disable-cache',
+      '--interpreter-constraint=>=2.7,<3',
+      # Add a constraint that's impossible to satisfy. Because multiple
+      # constraints OR, the interpeter should still resolve to Python 2.7.
+      '--interpreter-constraint=>=500',
+      '-o', pex_out_path])
+    res.assert_success()
+    pex_info = get_pex_info(pex_out_path)
+    assert {'>=2.7,<3', '>=500'} == set(pex_info.interpreter_constraints)
     assert pex_info.build_properties['version'][0] < 3
 
 
@@ -388,8 +402,7 @@ def test_interpreter_resolution_with_pex_python_path():
     pex_out_path = os.path.join(td, 'pex.pex')
     res = run_pex_command(['--disable-cache',
       '--rcfile=%s' % pexrc_path,
-      '--interpreter-constraint=%s' % interpreter_constraint1,
-      '--interpreter-constraint=%s' % interpreter_constraint2,
+      '--interpreter-constraint=%s,%s' % (interpreter_constraint1, interpreter_constraint2),
       '-o', pex_out_path])
     res.assert_success()
 
@@ -420,8 +433,7 @@ def test_interpreter_resolution_pex_python_path_precedence_over_pex_python():
     pex_out_path = os.path.join(td, 'pex.pex')
     res = run_pex_command(['--disable-cache',
       '--rcfile=%s' % pexrc_path,
-      '--interpreter-constraint=>3',
-      '--interpreter-constraint=<3.8',
+      '--interpreter-constraint=>3,<3.8',
       '-o', pex_out_path])
     res.assert_success()
 
@@ -515,8 +527,7 @@ def test_pex_python():
     pex_out_path = os.path.join(td, 'pex.pex')
     res = run_pex_command(['--disable-cache',
                            '--rcfile=%s' % pexrc_path,
-                           '--interpreter-constraint=>3',
-                           '--interpreter-constraint=<3.8',
+                           '--interpreter-constraint=>3,<3.8',
                            '-o', pex_out_path],
                           env=env)
     res.assert_success()
@@ -536,8 +547,7 @@ def test_pex_python():
     pex_out_path = os.path.join(td, 'pex2.pex')
     res = run_pex_command(['--disable-cache',
                            '--rcfile=%s' % pexrc_path,
-                           '--interpreter-constraint=>3',
-                           '--interpreter-constraint=<3.8',
+                           '--interpreter-constraint=>3,<3.8',
                            '-o', pex_out_path],
                           env=env)
     res.assert_success()