When a function accepts arguments, the names of those arguments become variables in the function's namespace. Whatever value was passed to the function when it was invoked becomes the value of those variables. Arguments can have default values, which makes them optional. If an argument is omitted, then its default value will be used. The following code defines a function called cap, which will check if a number is within an upper and lower limit. The default lower limit is 0, and the default upper limit is 100.
In Ignition, some complicated script functions are designed to take keyword arguments instead of normal parameters. In the description for those functions, you may see the following info box in this User Manual:
This function accepts keyword arguments.
Arguments can also be specified by keyword instead of by position. In the example above, the only way someone would know that the 200 in the last call to
cap specified the max is by its position. This can lead to hard-to-read function invocations for functions with lots of optional arguments. You can use keyword-style invocation to improve readability. The following code is equivalent to the last line above, using 200 for the
max and the default for the
Because we used a keyword to specify that 200 was the
max, we were able to omit the
min argument altogether, using its default. However, using a keyword argument before a non-keyword or positional argument is not allowed.
Functions Are Objects
Perhaps one of the most foreign concepts for new Python users is that in Python, functions are first-class objects. This means that functions can be passed around to other functions (this concept is similar to the idea of function pointers in C or C++).
Suppose we wanted a general way to filter a list. Maybe sometimes we want the odd entries, while other times we want even ones. We can define a function called
extract that takes a list and another function, and returns only entries that "pass" through the other function.
Where Can Functions Be Defined
User Defined Functions can be defined anywhere that a script is used. As stated before, they are useful to run segments of code multiple times without having to repeat it. They are also used extensively in project scripts where multiple functions can be defined in a single script module. Finally, some special Ignition System functions like system.gui.createPopupMenu or the runScript Expression function use functions as arguments.
The concept of scope is very important in all programming, and Python is no exception. Scope defines what names are directly accessible without any qualifiers. Another way to put this is that the scope determines what variables are defined. In Python, a variable is defined at the time that it is assigned. What scope it belongs to is also defined by where the assignment occurs.
When you define a function, that function gets its own scope. Variables that are assigned within that function body will not be available outside of the function.