2.3.2 Basic Commands

In this lab, we will introduce some simple Python commands. For more resources about Python in general, readers may want to consult the tutorial at docs.python.org/3/tutorial/.

Like most programming languages, Python uses functions to perform operations. To run a function called fun, we type fun(input1, input2), where the inputs (or arguments) input1 and input2 tell Python how to run the function. A function can have any number of inputs. For example, the print() function outputs a text representation of all of its arguments to the console.

In [1]: print('fit a model with', 11, 'variables')
fit a model with 11 variables

The following command will provide information about the print() function.

In [2]: print?

Adding two integers in Python is pretty intuitive.

In [3]: 3 + 5
Out[3]: 8

In Python, textual data is handled using strings. For instance, "hello" and 'hello' are strings. We can concatenate them using the addition + symbol.

In [4]: "hello" + " " + "world"
Out[4]: 'hello world'

A string is actually a type of sequence: this is a generic term for an ordered list. The three most important types of sequences are lists, tuples, and strings. We introduce lists now.

The following command instructs Python to join together the numbers 3, 4, and 5, and to save them as a list named x. When we type x, it gives us back the list.

In [5]: x = [3, 4, 5]
        x
Out[5]: [3, 4, 5]

Note that we used the brackets [] to construct this list.

We will often want to add two sets of numbers together. It is reasonable to try the following code, though it will not produce the desired results.

In [6]: y = [4, 9, 7]
        x + y
Out[6]: [3, 4, 5, 4, 9, 7]

The result may appear slightly counterintuitive: why did Python not add the entries of the lists element-by-element? In Python, lists hold arbitrary objects, and are added using concatenation. In fact, concatenation is the behavior that we saw earlier when we entered "hello" + " " + "world".

This example reflects the fact that Python is a general-purpose programming language. Much of Python’s data-specific functionality comes from other packages, notably numpy and pandas. In the next section, we will introduce the numpy package. See docs.scipy.org/doc/numpy/user/quickstart.html for more information about numpy.