The idioms of a programming language are defined by its users. Over the years, the Python community has come to use the adjective Pythonic to describe code that follows a particular style. The Pythonic style isn’t regimented or enforced by the compiler. It has emerged over time through experience using the language and working with others. Python programmers prefer to be explicit, to choose simple over complex, and to maximize readability (type import this).
Programmers familiar with other languages may try to write Python as if it’s C++, Java, or whatever they know best. New programmers may still be getting comfortable with the vast range of concepts expressible in Python. It’s important for everyone to know the best—the Pythonic—way to do the most common things in Python. These patterns will affect every program you write.