I would like to use PyCharm Community Edition 2020.1.2 x64 on Windows 10 as IDE for writing QGIS plugins. I need to base on old version - QGIS 2.18, so I need python 2.7, qt4, and also core modules from QGIS 2.18. I downloaded QGIS 2.18 from official site. It provides a ready to use python environment as .bat script, that sets all needed python paths. So I used path D:\Program Files\QGIS 2.18\bin\python-qgis-ltr.bat to set main python interpreter for my project.
After that I was able to run python console in PyCharm, import all QGIS modules, and any classes from QGIS standard collection like QgsFeature, QgsVectorLayer etc. Everything was fine untill I tried to import this classes from PyCharm editor in .py file as from qgis.core import QgsFeature. In editor PyCharm sees the package qgis.core and I'm able to import it, but PyCharm's code completion inside the module sees only classes that are saved in D:\Program Files\QGIS 2.18\apps\qgis-ltr\python\qgis\core\__init__.py e.g. QgsEditError class, but all standard QGIS class underlines on red.
Python console ran on exactly the same interpreter see the same sources as are visible in "External Libraries" tab in PyCharm, so command:
qgis.core.__path__returns['D:\\Program Files\\QGIS 2.18\\apps\\qgis-ltr\\python\\qgis\\core'],qgis.core.__file__returns'D:\\Program Files\\QGIS 2.18\\apps\\qgis-ltr\\python\\qgis\\core\\__init__.pyc,dir(qgis.core)returns full list of QGIS standard core classes.
If I run the .py script, in which PyCharm's code validator is showing incorrect import object by red underline, the script is executed without any error and I can easly print the imported QgsFeature object. Why my PyCharm's editor see different content of qgis.core module then console ran also from IDE on exactly the same interpreter?