In the past, I've successfully used arcpy.TestSchemaLock prior to undertaking schema modifications within script tools. I've recently upgraded to 10.5.1 and discovered that arcpy.TestSchemaLock is now returning False when it should return True.
The example snippet below returns False in the Python window, but I can immediately add a field to the tested feature class, which indicates that it is not locked and should have returned True.
Did something get broken in 10.5.1 that worked in 10.4.1?
>>> fc = r'C:\Workspace\2017_Hyde\incident_data\2017_Hyde_AzCof331_Events.gdb\EventLine
>>> canGetLock = arcpy.TestSchemaLock(fc)