Hello World

Lets get right to everyone's favorite example: the following script will print out "Hello World " to the output console .

The print keyword is a handy tool in Python, allowing you to put text into the output console. This is useful for debugging your scripts. You can print multiple things by separating them with commas.

Variables

Variables are created by simply assigning a value to them. Variables do not need to be declared, because Python has a dynamic type system. That means Python figures out the type of the variable on the fly, when the script is executed.

The following script would print out: 15

Quotations

Python makes limited distinction between single and double quotes. As long as they are used consistently then there are few times when which type of quotation mark you use matters. Some of the rules are shown here:

Strings, Numbers, and Booleans

Literal strings can be typed in using either double quotes or single quotes. This can be handy when your string contains one quote or the other. You can also use the backslash character to escape special characters.

Strings that contain characters beyond 7-bit ASCII, such as é or щ need to be marked as unicode strings by placing the letter u in front of the string. There is no harm in marking all strings as unicode strings. The following prints a unicode string:

Numbers can just be typed in normally, like 42 or 3.14159. Python does not have a boolean type. 0 is false and 1 is true. For convenience, Python also has constants named False and True which can be used in place of 0 and 1. This is an oversimplification, but should suffice for now. The following prints out "True".

None

There is a special value in Python called None (with a capital N). This is simply a special value that means: no value. This value is equivalent to Java's null value.

List

In Python, lists (arrays) are a built-in type that contains multiple other values. Lists can contain any type of items, and the items in a list do not all need to be the same type. You can create a list by enclosing multiple items in square brackets ( [] ), separated with commas. You can pull items out of a list with the square-bracket list index notation. Note that lists are zero-indexed, meaning that the first item in the list is at position 0. This code will print out " a list ".

Basic Operators

Python has all of the normal arithmetic operators you'd expect, addition(+), subtraction(-), division(/), multiplication(*), modulus(%), etc.

The comparison operators are just like in C: equals(==), not equals(!=) greater than (>), greater than or equal(>=), etc.

The logical operators are just typed in plain text: and, or, not.

Comments

Comments start with a hash sign. Add comments to your code so that when you go back to it after a long time, you know what the code is trying to do.

Whitespace

Perhaps its most unique feature, logical blocks are defined by indentation in Python. A colon ( : ) starts a new block, and the next line must be indented (typically using a tab of 4 spaces). The block ends when the indentation level returns to the previous level. For example, the following will print out " 5 4 3 2 1 Blast-off ". The final print is not part of the loop, because it isn't indented.

if Statement

The if statement is familiar to anyone with a passing knowledge of programming. The idea of an if is that you want your script to execute a block of statements only if a certain condition is true. For example, this script won't do anything.

You can use the if...else form of an if statement to do one thing if a condition is true, and something else if the condition is false. This script will print out " this will show!"

Lastly, you can use the if...elif form. This form combines multiple condition checks. elif stands for else if. This form can optionally have a catch-all else clause at the end. For example, this script will print out "three":

while Loops

A while loop will repeat a block of statements while a condition is true. This code will print out the contents of the items in the list. This code uses a function called len , which is a built-in function that returns the length of a list or string.

for Loops

Python's for loop may be a bit different than what you're used to if you've programmed any C. The for loop is specialized to iterate over the elements of any sequence, like a list. So, we could re-write the example above using a for loop eliminating the counter x:

Much more graceful! You'll often see the for loop used instead of the while loop, even when you simply want to iterate a given number of times. To do this with the for loop, you can use the built-in function range. The range function returns a variable-size list of integers starting at zero. Calling range(4) will return the list [0, 1, 2, 3]. So, to have a for loop repeat 4 times, you can simply do:

break and continue in Loops

You can stop a loop from repeating in its tracks by using the break statement. This code will print out " Loop " exactly two times, and then print " Finished ".

You can use the continue statement to make a loop stop executing its current iteration and skip to the next one. The following code will print out the numbers 0-9, skipping 4.

Defining Functions

Functions are defined using the def keyword. A function needs a name and a list of the arguments that it can be passed. For example, this code defines a function that tests whether or not a number is odd. It returns a true value (1) if the number is odd. It is then used in a loop to print out the odd numbers between 0 and 9.

Functions Arguments

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. In the example above, the value of x inside the for loop gets passed to the isOdd function, and becomes the value of the num argument.

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 calledcap, which will take a number, and make sure it is within an upper and lower limit. The limits default to 0 and 100.

Keyword Arguments

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:

Arguments can also be specified by keyword instead of by position. In the example below, 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 min.

Because we used a keyword to specify that 200 was the max, we were able to omit the min argument altogether, using its default.

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++).

Lets go back to the isOdd example above. Suppose we wanted a more general way to filter a list. Maybe sometimes we want the odd entries, while other times we want even ones, or entries less than 3, etc. We can define a function called extract that takes a list and another function, and returns only entries that "pass" through the other function.

Now, it just so happens that Python has a built-in function that does exactly what our extract function does - its called filter.

We would also be remiss at this point if we didn't mention another language feature called list comprehensions. This is a great little bit of syntax that helps make new lists out of other lists. Instead of using our filter function, we could have simply done this:

In Ignition, you'll most commonly see functions used as objects when using the system.util.invokeLater function. This function takes a function and executes it after all pending event handling has finished processing.

Definition by Assignment

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.

Function Scope

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.

Import Keyword

You can import the namespaces defined in other scopes into your scope with the import keyword. Most commonly, you'll import from global library sources, like system (the Ignition standard libraries), project (your project's script library), java (importing from the Java standard library), and the various python standard libraries. When you're writing component event handlers, system, shared, and project are imported for you automatically.

The import keyword can be used in a variety of forms:

• import X

• from X import Y

For example, suppose you wanted to use the java.util.Calendar class for some date manipulations. You could import this in a number of different ways. These examples are equivalent, printing out a date 8 hours before the current date.

List

Lists are a very handy kind of sequence. They can hold any number of items and can be resized on the fly. After creating a list using the square bracket notation, there are a number of functions that you can call on the list. Some common list functions are listed here.

append(x)Takes a single argument, which will be appended to the end of the list.

insert(i,x)Inserts an item x at index i

remove(x)Will remove the given item from the list.

index(x)Returns the index of the value x. Throws an error if the list doesn't contain the item. Use the in operator to check if an item is contained in a sequence.

sort()Sorts the items in the list.

Tuples

A tuple is similar to a list, but you use parenthesis instead of square brackets to define one. The major difference between a tuple and a list is that tuple's are immutable. That is, once created, they cannot be altered. Tuples are very useful for passing multiple things to and from functions. For example, you could pass a point to a function using a tuple like so:

This can also be handy for returning multiple values from a function. For example, if you had a mouse event, you could write a function that found the component's center point, and return that point as a tuple. You could then use unpacking assignment to extract the values into separate values.

Dictionaries

A dictionary is a very useful type that holds a set of key-value pairs. You may have used these in other languages and know them as hashmaps, maps, associative memories or associative arrays. Dictionaries are not ordered sequences - you reference any item via its key value. The keys can be numbers, strings, or tuples of these types. Any given key may only appear once in a dictionary. Trying to set another value for that key will overwrite any previous value for that key.

Dictionaries are created using braces ({}). Key-value pairs are separated by commas, and the keys are separated from the values with a colon. You can use the .keys() function to have a set of the keys. For example:

You can loop through dictionaries using a for loop. You can use the keys() to loop through the dictionary, and then use the key values to look up the value. For example: