To view the list of file types recognized by PyCharm, go to Settings/Preferences (Ctrl+Alt+S) . If a file in your project is marked with the icon, it indicates that PyCharm can't recognize it. In this case, you can register and configure a new file type.
The lock decorator in the upper-left part of a symbol marks a symbol whose name begins with one or more underscores. Such names are considered pseudo-private and are specially treated by an interpreter to restrict their visibility scope.
The blue bubble in the upper-left part of a symbol marks a symbol whose name begins and ends with two underscores, and is specially treated by an interpreter and standard library.
Anonymous bookmark – a check sign bookmark. Bookmarks are used for quick navigation within a file or across the entire project.
Lettered mnemonic bookmark – a letter in the interval from A to Z.
Numbered mnemonic bookmark – a number in the interval from 0 to 9.
DB data source. Also, DBMS-specific icons are used:
DB data source with the read-only status, e.g. for Derby.
DDL data source
Column with a primary key
Column with a foreign key
Column with an index
Stored procedure or function