Display warning if Python version is less than 3.7 (#994)

* Add Python version check and faulty test to confirm it's working (should fail on 3.6 build only)
* Apply formatting
* Fix behave Python version test
* Make error message more descriptive and friendly

features/core.feature

@@ -135,3 +135,7 @@ Feature: Basic reading and writing to a journal
         When we run "jrnl --diagnostic"
         Then the output should contain "jrnl"
         And the output should contain "Python"
+
+    Scenario: Version warning appears for versions below 3.7
+        When we run "jrnl --diagnostic"
+        Then the Python version warning should appear if our version is below 3.7

features/steps/core.py

@@ -372,6 +372,20 @@ def list_journal_directory(context, journal="default"):
             print(os.path.join(root, file))
 
 
+@then("the Python version warning should appear if our version is below {version}")
+def check_python_warning_if_version_low_enough(context, version):
+    import packaging.version
+    import platform
+
+    if packaging.version.parse(platform.python_version()) < packaging.version.parse(
+        version
+    ):
+        out = context.stderr_capture.getvalue()
+        assert "WARNING: Python versions" in out
+    else:
+        assert True
+
+
 @then("fail")
 def debug_fail(context):
     assert False

jrnl/cli.py

@@ -21,6 +21,7 @@
 
 import argparse
 import logging
+import packaging.version
 import platform
 import re
 import sys
@@ -29,7 +30,7 @@ import jrnl
 
 from . import install, plugins, util
 from .Journal import PlainJournal, open_journal
-from .util import ERROR_COLOR, RESET_COLOR, UserAbort
+from .util import WARNING_COLOR, ERROR_COLOR, RESET_COLOR, UserAbort
 
 log = logging.getLogger(__name__)
 logging.getLogger("keyring.backend").setLevel(logging.ERROR)
@@ -313,6 +314,18 @@ def configure_logger(debug=False):
 
 
 def run(manual_args=None):
+    if packaging.version.parse(platform.python_version()) < packaging.version.parse(
+        "3.7"
+    ):
+        print(
+            f"""{WARNING_COLOR}
+WARNING: Python versions below 3.7 will no longer be supported as of jrnl v2.5
+(the next release). You are currently on Python {platform.python_version()}. Please update to
+Python 3.7 (or higher) soon.
+{RESET_COLOR}""",
+            file=sys.stderr,
+        )
+
     if manual_args is None:
         manual_args = sys.argv[1:]