Fix Python – What is the naming convention in Python for variable and function?

Coming from a C# background the naming convention for variables and method names are usually either camelCase or PascalCase:
// C# example
string thisIsMyVariable = “a”
public void ThisIsMyMethod()

In Python, I have seen the above but I have also seen underscores being used:
# python example
this_is_my_variable = ‘a’
def this_is_my_function():