__init__.py file?A Python package usually consists of several code files. Physically, a package is a folder containing files and maybe other folders that themselves may contain more folders and files. Conceptually, it’s a namespace. This simply means that a package’s modules are bound together by a package name, by which they may be referenced.
Since a Python module is simply reusable, importable code, we can consider every package to be a module — but not every module is a package. A package folder usually contains one file named __init__.py that basically tells Python that the directory is a package. The __init__.py file may be empty, or it may contain code to be executed upon package initialization. In other words, if an __init__.py file is present in a package directory, it will be invoked when the package or a module within the package is imported into another file. So any code that is inside an __init__.py file can be used to execute package initialization code, such as initialization of package-level data.
Note that if you run a file as a standalone script, then any __init__.py files that are in the same package will NOT be executed. The __init__.py files are only executed when the package or a module within the package is imported into another file.
__init__.py files required to create a package?Much of the Python documentation states that an __init__.py file must be present in the package directory when creating a package. This was once true. It used to be that the very presence of __init__.py signified to Python that a package was being defined. The file could contain initialization code or even be empty, but it had to be present.
Starting with Python 3.3, Implicit Namespace Packages were introduced. These allow for the creation of a package without any __init__.py file. Of course, it can still be present if package initialization is needed. But it is no longer required. Check out What’s a Python Namespace Package, and What’s It For? to learn more.