Python interpreter step by step

Learning Python — step by step — 2 — the interpreter

In the previous post we installed Python and checked that we had the correct version installed. In this post we’ll start diving into some basic python coding.

Starting of really easy

Let’s start a python interpreter like we did yesterday. Here we can play around with some basic code to learn more about python strings. Strings are words, sentences or just simple characters.

Open up a command line, type «python» and hit enter. You should see something like this:

$ python Python 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 23:03:10) [MSC v.1916 64 bit (AMD)] on win32 Type "help", "copyright, "credits" or "license" for more information. >>>

To print information to the screen, we use something called «print». You can think of «print» as a function you give information and in return, «print» will show the information you give it on the screen.

>>> print("Hello") Hello >>> print('Python rocks') Python rocks

This might not be super exciting, but at least it show you how to print information on the screen.

If you look close, you can see that «Hello» has double quotes and ‘Python rocks’ has single quotes. You can use both, but I suggest that you stick to one of them to make your code as clean as possible.

String

Both «Hello» and ‘Python rocks’ are examples of strings. But they’re not doing anything special and since we just put them right in the «print» function, we can’t actually do anything with them.

To make strings a little bit more exciting, we can store them to something called a variable. You can think of a variable as a drawer, you assign it a spot (a name if our case) and you store something in them. You can store strings, numbers, objects, etc.

Below the line where we printed that Python rocks, we are going to create a variable. Type the following code and hit enter:

We now have a variable called «name» which contains a string with this value ‘Stein Ove’. We can do a lot of things with this string. Let’s first try to just print it to the screen.

There’s nothing new about this, it’s just that we print a variable instead of a static string like before.

Print a greeting

We have a name stored in a variable. Let’s print a greeting to this person.

>>> print("Hello " + name) Hello Stein Ove

In this little snippet, we used the «print» function to combine a static string «Hello» with the variable. Let’s try to add «Hello» to the variable and print it that way.

>>> greeting = "Hello " + name >>> print(greeting) Hello Stein Ove

This gives the same result, but the way we do it is a little bit different.

Let’s try a little example where we combine a string with a number.

>>> age = 32 >>> sentence = 'Age: ' + age

If you try to run this code, you will get an error. This is because you can’t concatenate a string and a number. So what do you do when you want to print something like this?

Luckily, there’s multiple ways of concatenating strings in python. Let’s create a new «sentence» variable:

>>> sentence = 'Age:', age >>> print(sentence) Age: 32

As you can see, you’ll get the correct output now. You should also notice that when you use «,» to concatenate strings, you don’t need a space before the quote like you did earlier.

Summary

Now you should be able to create different sentences, concatenate words with numbers and so on. Play around a little bit to become more familiar with this before you continue.

Here is a couple of more examples:

$ python Python 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 23:03:10) [MSC v.1916 64 bit (AMD)] on win32 Type "help", "copyright, "credits" or "license" for more information. >>> print('Stein Ove') Stein Ove >>> name = 'Stein Ove' >>> print(name) Stein Ove >>> age = 32 >>> print('Stein Ove is', age, 'years old') Stein Ove is 32 years old >>> print('Hello ' + name) Hello Stein Ove

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2. Using the Python Interpreter¶

The Python interpreter is usually installed as /usr/local/bin/python3.11 on those machines where it is available; putting /usr/local/bin in your Unix shell’s search path makes it possible to start it by typing the command:

to the shell. 1 Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g., /usr/local/python is a popular alternative location.)

On Windows machines where you have installed Python from the Microsoft Store , the python3.11 command will be available. If you have the py.exe launcher installed, you can use the py command. See Excursus: Setting environment variables for other ways to launch Python.

Typing an end-of-file character ( Control — D on Unix, Control — Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn’t work, you can exit the interpreter by typing the following command: quit() .

The interpreter’s line-editing features include interactive editing, history substitution and code completion on systems that support the GNU Readline library. Perhaps the quickest check to see whether command line editing is supported is typing Control — P to the first Python prompt you get. If it beeps, you have command line editing; see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. If nothing appears to happen, or if ^P is echoed, command line editing isn’t available; you’ll only be able to use backspace to remove characters from the current line.

The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a script from that file.

A second way of starting the interpreter is python -c command [arg] . , which executes the statement(s) in command, analogous to the shell’s -c option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote command in its entirety.

Some Python modules are also useful as scripts. These can be invoked using python -m module [arg] . , which executes the source file for module as if you had spelled out its full name on the command line.

When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing -i before the script.

All command line options are described in Command line and environment .

2.1.1. Argument Passing¶

When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the argv variable in the sys module. You can access this list by executing import sys . The length of the list is at least one; when no script and no arguments are given, sys.argv[0] is an empty string. When the script name is given as ‘-‘ (meaning standard input), sys.argv[0] is set to ‘-‘ . When -c command is used, sys.argv[0] is set to ‘-c’ . When -m module is used, sys.argv[0] is set to the full name of the located module. Options found after -c command or -m module are not consumed by the Python interpreter’s option processing but left in sys.argv for the command or module to handle.

2.1.2. Interactive Mode¶

When commands are read from a tty, the interpreter is said to be in interactive mode. In this mode it prompts for the next command with the primary prompt, usually three greater-than signs ( >>> ); for continuation lines it prompts with the secondary prompt, by default three dots ( . ). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:

$ python3.11 Python 3.11 (default, April 4 2021, 09:25:04) [GCC 10.2.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> 

Continuation lines are needed when entering a multi-line construct. As an example, take a look at this if statement:

>>> the_world_is_flat = True >>> if the_world_is_flat: . print("Be careful not to fall off!") . Be careful not to fall off! 

For more on interactive mode, see Interactive Mode .

2.2. The Interpreter and Its Environment¶

2.2.1. Source Code Encoding¶

By default, Python source files are treated as encoded in UTF-8. In that encoding, characters of most languages in the world can be used simultaneously in string literals, identifiers and comments — although the standard library only uses ASCII characters for identifiers, a convention that any portable code should follow. To display all these characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file.

To declare an encoding other than the default one, a special comment line should be added as the first line of the file. The syntax is as follows:

where encoding is one of the valid codecs supported by Python.

For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:

One exception to the first line rule is when the source code starts with a UNIX “shebang” line . In this case, the encoding declaration should be added as the second line of the file. For example:

#!/usr/bin/env python3 # -*- coding: cp1252 -*- 

On Unix, the Python 3.x interpreter is by default not installed with the executable named python , so that it does not conflict with a simultaneously installed Python 2.x executable.

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