Python Articles

Cheat Sheet / Updated 04-12-2024

Python is a flexible programming language that has become increasingly popular in the past few years. This cheat sheet is designed to give you a handy resource for common Python data types, Python operators, and Python functions. It includes Python data types, operators, special characters, f-strings, and functions for working with robots.

Article / Updated 10-04-2023

Programming is an important skill. Python will serve you well for years to come. The tables here give you the core words, built-ins, standard library functions, and operators that you'll use most when you're coding with Python. Python Core Words KeywordSummaryExample and Logical operator to test whether two things are both True. andx>2 and x<10 as Assign a file object to a variable. Used with with.Let your code refer to a module under a different name (also called an alias). Used with import. with open(<name of file>,<file mode>) as <object name>:import cPickle as pickle break Stop execution of a loop. for i in range(10): if i%2 ==0: break class Define a custom object. class <name of class>(object): ""Your docstring"" class MyClass(object): ""A cool function."" continue Skip balance of loop and begin a new iteration. for i in range(10): if i%2 ==0: continue def Define a function. def <name of function>(): ""Your docstring"" def my_function(): ""This does... "" elif Add conditional test to an if clause. See if. else Add an alternative code block. See if. for Create a loop which iterates through elements of a list (or other iterable). for <dummy variable name> in <sequence>:for i in range(10): from Import specific functions from a module without importing the whole module. from <module name> import <name of function or object>from random import randint global Make a variable global in scope. (If a variable is defined in the main section, you can change its value within a function.) global x if Create a condition. If the condition is True, the associated code block is executed. Otherwise, any elif commands are processed. If there are none, or none are satisfied, execute the else block if there is one. if : [elif : , ...][else: ]if x == 1: print("x is 1")elif x == 2: print("x is 2")elif x > 3: print("x is greater than 3")else print("x is not greater than 3, nor is it 1 one or 2") import Use code defined in another file without retyping it. import <name of module>import random in Used to test whether a given value is one of the elements of an object. 1 in range(10) is Used to test whether names reference the same object. x = Nonex is None # faster thanx == None lambda Shorthand function definition. Usually used where a function needs to be passed as an argument to another function. lamda :times = lambda x, y: x*ycommand=lambda x: self.draw_line(self.control_points) not Logical negation, used to negate a logical condition. Don't use for testing greater than, less than, or equal. 10 not in range(10) or Logical operator to test whether at least one of two things is True. orx<2 or x>10 pass Placeholder keyword. Does nothing but stop Python complaining that a code block is empty. for i in range (10): pass print Output text to a terminal. print("Hello World!") return Return from the execution of a function. If a value is specified, return that value, otherwise return None. return return x+2 while Execute a code block while the associated condition is True. while :while True: pass with Get Python to manage a resource (like a file) for you. with open(,) as : Extend Python's core functionality with these built-ins. Python Built-ins Built-inNotesExample False Value, returned by a logical operation or directly assigned. ok_to_continue = Falseage = 16old_enough = age >=21(evaluates comparison age>=21 and assigns the result to old_enough) None Value used when representing the absence of a value or to initialise a variable which will be changed later. Returned by functions which do not explicitly return a value. x = None True Value, returned by a logical operation. ok_to_continue = Trueage = 16old_enough = age >=21(evaluates comparison age>=21 and assigns the result to old_enough) __name__ Constant, shows module name. If it's not "__main__", the code is being used in an import. if __name__=="__main__": dir List attributes of an item. dir(<object name>) enumerate Iterate through a sequence and number each item. enumerate('Hello') exit Exit Python (Command Line) interpreter. exit() float Convert a number into a decimal, usually so that division works properly. 1/float(2) getattr Get an attribute of an object by a name. Useful for introspection. getattr(<name of object>, <name of attribute>) help Get Python docstring on object. help(<name of object>)help(getattr) id Show the location in the computer's RAM where an object is stored. id(<name of object>)id(help) int Convert a string into an integer number. int('0') len Get the number of elements in a sequence. len([0,1]) object A base on which other classes can inherit from. class CustomObject(object): open Open a file on disk, return a file object. open(, )open('mydatafile.txt', 'r') # read(opens a file to read data from)open('mydatafile.txt', 'w') # write(creates a new file to write to, destroys any existing file with the same name)open('mydatafile.txt', 'a') # append(adds to an existing file if any, or createsa new one if none existing already) print Reimplementation of print keyword, but as a function.Need to import from the future to use it (srsly!) from future import print_functionprint ('Hello World!') range Gives numbers between the lower and upper limits specified (including the lower, but excluding the upper limit). A step may be specified. range(10)range(5,10)range(1,10,2) raw_input Get some text as a string from the user, with an optional prompt. prompt = 'What is your guess? 'players_guess = raw_input(prompt) str Convert an object (usually a number) into a string (usually for printing). str(0) type Give the type of the specified object. type(0)type('0')type([])type({})type(()) Use the work that others have already done. Try these modules from the Python standard library. Selected Functions from the Standard Library ModuleWhat It DoesSample Functions/Objects os.path Functions relating to files and file paths. os.path.exists() pickle, cPickle Save and load objects to/from a file. pickle.load(), pickle.dump(, ) random Various functions relating to random numbers. random.choice(), random.randint(, ), random.shuffle() String Stuff relating to strings. string.printable sys Various functions related to your computer system. sys.exit() Time Time-related functions. time.time() Tkinter User interface widgets and associated constants. Tkinter.ALLTkinter.BOTHTkinter.CENTERTkinter.ENDTkinter.HORIZONTALTkinter.LEFTTkinter.NWTkinter.RIGHTTkinter.TOPTkinter.YTkinter.Button(,text=)Tkinter.Canvas(, width=, height=)Tkinter.Checkbutton(, text=)Tkinter.Entry(, width=),Tkinter.Frame()Tkinter.IntVar()Tkinter.Label(, text = )Tkinter.mainloop()Tkinter.Menu()Tkinter.OptionMenu(, None, None)Tkinter.Scale(, from_=, to=)Tkinter.Scrollbar()Tkinter.StringVar()Tkinter.Tk() Add, subtract, divide, multiply, and more using these operators. Python Operators OperatorNameEffectExamples + Plus Add two numbers.Join two strings together. Add: >>> 1+12Join: >>> 'a'+'b''ab' – Minus Subtract a number from another.Can't use for strings. >>> 1-10 * Times Multiply two numbers.Make copies of a string. Multiply: >>> 2*24Copy: >>> 'a'*2'aa' / Divide Divide one number by another.Can't use for strings. 1/2 # integer division:Answer will be rounded down.1/2.0 # decimal division1/float(2) # decimal division % Remainder (Modulo) Give the remainder when dividing the left number by the right number.Formatting operator for strings. >>> 10%31 ** Power x**y means raise x to the power of y.Can't use for strings. >>> 3**29 = Assignment Assign the value on the right to the variable on the left. >>> a = 1 == Equality Is the left side equal to the right side? Is True if so; is False otherwise. >>> 1 == 1True>>> 'a' == 'a'True != Not equal Is the left side not equal to the right side? Is True if so; is False otherwise. >>> 1 != 1False>>> 1 != 2True>>> 'a' != 'a'True > Greater than Is the left side greater than the right side?>= means greater than or equal to >>> 2 > 1True < Less than Is the left side less than the right side?<= means less than or equal to >>> 1 < 2True & (or and) And Are both left and right True?Typically used for complex conditions where you want to do something if everything is True:while im_hungry and you_have_food: >>> True & TrueTrue>>> True and FalseFalse >>> True & (1 == 2)False | (or or) Or Is either left or right True?Typically used for complex conditions where you want at least one thing to be True:while im_bored or youre_bored: >>> True | FalseTrue>>> True or FalseTrue>>> False | FalseFalse>>> (1 == 1) | FalseTrue

Cheat Sheet / Updated 10-03-2023

Python is an incredible programming language that you can use to perform data science tasks with a minimum of effort. The huge number of available libraries means that the low-level code you normally need to write is likely already available from some other source. All you need to focus on is getting the job done. With that in mind, this Cheat Sheet helps you access the most commonly needed reminders for making your programming experience fast and easy.

Article / Updated 09-13-2023

Many organizations are using Python these days to perform major tasks. You don't necessarily hear about them because organizations are usually reserved about giving out their trade secrets. However, Python is still there making a big difference in the way organizations work and toward keeping the bottom line from bottoming out. Following, are some major ways in which Python is used commercially that will make it easier to argue for using Python in your own organization. (Or you can read about some Python success stories.) Corel: PaintShop Pro is a product that many people have used over the years to grab screenshots, modify their pictures, draw new images, and perform a lot of other graphics-oriented tasks. The amazing thing about this product is that it relies heavily on Python scripting. In other words, to automate tasks in PaintShop Pro, you need to know Python. D-Link: Upgrading firmware over a network connection can be problematic, and D-Link was encountering a situation in which each upgrade was tying up a machine — a poor use of resources. In addition, some upgrades required additional work because of problems with the target device. Using Python to create a multithreaded application to drive updates to the devices allows one machine to service multiple devices, and a new methodology allowed by Python reduces the number of reboots to just one after that new firmware is installed. D-Link chose Python over other languages, such as Java, because it provides an easier-to-use serial communication code. Eve-Online: Games are a major business because so many people enjoy playing them. Eve-Online is a Massively Multiplayer Online Role Playing Game (MMORPG) that relies heavily on Python for both the client and server ends of the game. It actually relies on a Python variant named StacklessPython, which is important because you encounter these variants all the time when working with Python. Think of them as Python on steroids. These variants have all the advantages of Python, plus a few extra perks. The thing to take away from this particular company is that running an MMORPG takes major horsepower, and the company wouldn't have chosen Python unless it were actually up to the task. ForecastWatch.com: If you have ever wondered whether someone reviews the performance of your weatherman, look no further than ForecastWatch.com. This company compares the forecasts produced by thousands of weather forecasters each day against actual climatological data to determine their accuracy. The resulting reports are used to help improve weather forecasts. In this case, the software used to make the comparisons is written in pure Python because it comes with standard libraries useful in collecting, parsing, and storing data from online sources. In addition, Python's enhanced multithreading capabilities makes it possible to collect the forecasts from around 5,000 online sources each day. Most important of all, the code is much smaller than would have been needed by other languages such as Java or PHP. Frequentis: The next time you fly somewhere, you might be relying on Python to get you to the ground safely again. It turns out that Frequentis is the originator of TAPTools, a software product that is used for air traffic control in many airports. This particular tool provides updates on the weather and runway conditions to air traffic controllers. Honeywell: Documenting large systems is expensive and error prone. Honeywell uses Python to perform automated testing of applications, but it also uses Python to control a cooperative environment between applications used to generate documentation for the applications. The result is that Python helps generate the reports that form the documentation for the setup. Industrial Light & Magic: In this case, you find Python used in the production process for scripting complex, computer graphic-intensive films. Originally, Industrial Light & Magic relied on Unix shell scripting, but it was found that this solution just couldn't do the job. Python was compared to other languages, such as Tcl and Perl, and chosen because it's an easier-to-learn language that the organization can implement incrementally. In addition, Python can be embedded within a larger software system as a scripting language, even if the system is written in a language such as C/C++. It turns out that Python can successfully interact with these other languages in situations in which some languages can't. Philips: Automation is essential in the semiconductor industry, so imagine trying to coordinate the effort of thousands of robots. After a number of solutions, Philips decided to go with Python for the sequencing language (the language that tells what steps each robot should take). The low-level code is written in C++, which is another reason to use Python, because Python works well with C++.

Article / Updated 08-10-2023

Tradition dictates that Hello World! be the first program that you write when you're learning a new programming language like Python. You're following in the footsteps of many great programmers when you create this project. To create your Hello World! program, follow these steps: Open your Start menu and choose Python (command line). You should get a prompt that looks like >>>. At the moment, you're doing everything in interactive mode in the Python interpreter. That's where the >>> comes in. Python shows you >>> when you're supposed to type something. At the prompt, type the following. Use a single quote at the start and the end — it's beside the Enter key: print('Hello World!') Press the Enter key. Python runs the code you typed. You see the output shown in Figure 1. Congratulations — you've written your first program. Welcome to the Python-programmers-in-training club. If you don't see what's in Figure 1, check that you typed in the text from Step 2 exactly as it's written: Check that the parentheses and single quotes are in the right places. Check that for each opening parenthesis there is a closing parenthesis. (Otherwise, you're left hanging. Check that for each opening quote there's a closing quote. Programming languages have their own grammar and punctuation rules. These rules are the language's syntax. Humans, can work most stuff out even if perfect not you're is grammar (See? You figured out what that sentence was trying to say), but Python pretty much freaks out if you get the syntax wrong.

Article / Updated 08-10-2023

The Python interpreter takes in each line and operates on it immediately (more or less) after you press the Enter key. In Hello World! you use Python's print feature. print takes what's inside the parentheses and outputs it to the command line (also called the console). Python is sensitive to both the grammar and punctuation. If you misspell something, the program won't work. If Python is expecting special characters and you don't put them in, then Python will fail. Some Python issues are shown here. Can you work out how you would fix them? >>> pritn('Hello World!') Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'pritn' is not defined Here's another: >>> print('Hello World!) File "<stdin>", line 1 print('Hello World!) SyntaxError: EOL while scanning string literal Here's another: >>> print 'Hello World!') File "<stdin>", line 1 print 'Hello World!') ^ SyntaxError: invalid syntax Python tries to give you the reason it failed (that is, NameError and SyntaxError). Check each of these things: Opening parenthesis has a closing parenthesis Every opening quote mark has a matching closing quote mark All commands are correctly spelled

Article / Updated 07-10-2023

Before you can get going here, make sure you have your Raspberry Pi computer set up and running on your monitor, keyboard, and mouse. If not, go do that now The next few paragraphs are going to be lots more fun with a computer to work with! The Raspberry Pi is the perfect platform to do physical computing with Python because it has a multiscreen environment, lots of RAM and storage to play with and all the tools to build fun projects. A huge and powerful feature of the Raspberry Pi is the row of GPIO (general purpose input-output) pins along the top of the Raspberry Pi. It is a 40-pin header into which you can plug a large number of sensors and controllers to do amazing things to expand your Raspberry Pi. GPIO pins on the Raspberry Pi GPIO pins can be designated (using Python software) as input or output pins and used for many different purposes. There are two 5V power pins, two 3.3V power pins and a number of ground pins that have fixed uses. An GPIO pin output pin “outputs” a 1 or a 0 from the computer to the pin. Basically, A “1” is 3.3V and a “0” is 0V. You can think of them just as 1s and 0s. GPIO Python libraries There are a number of GPIO Python libraries that are usable for building projects. A good one to use is the gpiozero library that is installed on all Raspberry Pi desktop software releases. Check out the library documentation and installation instructions (if needed). Now let’s jump into the “Hello World” physical computing project with our Raspberry Pi. The hardware for “Hello World” on the Raspberry Pi To do this project, you’ll need some hardware. Because these instructions involve using Grove connectors, let's get the two pieces of Grove hardware that you need for this project: Pi2Grover: This converts the Raspberry Pi GPIO pin header to Grove connectors (ease of use and can’t reverse the power pins!). You can buy this either at shop.switchdoc.com or at Amazon.com. You can get $5.00 off the Pi2Grover board at shop.switchdoc.com by using the discount code PI2DUMMIES at checkout. Grove blue LED: A Grove blue LED module including Grove cable. You can buy this on shop.switchdoc.com or on amazon.com. How to assemble the Raspberry Pi’s hardware For a number of you readers, this will be the first time you have ever assembled a physical computer-based product. because of this, we’ll give you the step-by-step process: Identify the Pi2Grover board. Making sure you align the pins correctly gently press the Pi2Grover Board (Part A) onto the 40 pin GPIO connector on the Raspberry Pi. Gently finish pushing the Pi2Grover (Part A) onto the Raspberry Pi GPIO pins, making sure the pins are aligned. There will be no pins showing on either end and make sure no pins on the Raspberry Pi are bent. Plug one end of the Grove cable into the Grove blue LED board. If your blue LED is not plugged into the Grove blue LED board, then plug in the LED with the flat side aligned with the flat side of the outline on the board. Plug the other end of the Grove cable into the slot marked D12/D13 on the Pi2Grover board.You are now finished assembling the hardware. Now it’s time for the Python software.

Article / Updated 07-07-2023

You don’t need to understand absolutely every detail about how permanent storage works with Python in order to use it. For example, just how the drive spins (assuming that it spins at all) is unimportant. However, most platforms adhere to a basic set of principles when it comes to permanent storage. These principles have developed over a period of time, starting with mainframe systems in the earliest days of computing. Data is generally stored in files (with pure data representing application state information), but you could also find it stored as objects (a method of storing serialized class instances). probably know about files already because almost every useful application out there relies on them. For example, when you open a document in your word processor, you’re actually opening a data file containing the words that you or someone else has typed. Files typically have an extension associated with them that defines the file type. The extension is generally standardized for any given application and is separated from the filename by a period, such as MyData.txt. In this case, .txt is the file extension, and you probably have an application on your machine for opening such files. In fact, you can likely choose from a number of applications to perform the task because the .txt file extension is relatively common. Internally, files structure the data in some specific manner to make it easy to write and read data to and from the file. Any application you write must know about the file structure in order to interact with the data the file contains. File structures can become quite complex. Files would be nearly impossible to find if you placed them all in the same location on the hard drive. Consequently, files are organized into directories. Many newer computer systems also use the term folder for this organizational feature of permanent storage. No matter what you call it, permanent storage relies on directories to help organize the data and make individual files significantly easier to find. To find a particular file so that you can open it and interact with the data it contains, you must know which directory holds the file. Directories are arranged in hierarchies that begin at the uppermost level of the hard drive. For example, when working with the downloadable source code for this book, you find the code for the entire book in the BPPD directory within the user folder on your system. On a Windows system, that directory hierarchy is C:\Users\John\BPPD. However, other Mac and Linux systems have a different directory hierarchy to reach the same BPPD directory, and the directory hierarchy on your system will be different as well. Notice that you use a backslash (\) to separate the directory levels. Some platforms use the forward slash (/); others use the backslash. The book uses backslashes when appropriate and assumes that you'll make any required changes for your platform. A final consideration for Python developers (at least for this book) is that the hierarchy of directories is called a path. You see the term path in a few places in this book because Python must be able to find any resources you want to use based on the path you provide. For example, C:\Users\John\BPPD is the complete path to the source code on a Windows system. A path that traces the entire route that Python must search is called an absolute path. An incomplete path that traces the route to a resource using the current directory as a starting point is called a relative path. To find a location using a relative path, you commonly use the current directory as the starting point. For example, BPPD\__pycache__ would be the relative path to the Python cache. Note that it has no drive letter or beginning backslash. However, sometimes you must add to the starting point in specific ways to define a relative path. Most platforms define these special relative path character sets: \: The root directory of the current drive. The drive is relative, but the path begins at the root, the uppermost part, of that drive. .\: The current directory. You use this shorthand for the current directory when the current directory name isn't known. For example, you could also define the location of the Python cache as .\__pycache__. ..\: The parent directory. You use this shorthand when the parent directory name isn't known. ..\..\: The parent of the parent directory. You can proceed up the hierarchy of directories as far as necessary to locate a particular starting point before you drill back down the hierarchy to a new location.

Article / Updated 10-19-2022

Whether you use a Mac, Windows, or Linux OS (operating system), you can find and install Python on your computer. The following sections give you instructions for each OS. How to install Python on Mac OSX To find and start Python on Mac OSX computers, follow these steps: Press Cmd+spacebar to open Spotlight. Type the word terminal. Or, from the Finder, select Finder→Go→Utilities→Terminal. The Terminal window opens. In the terminal, type python. The Python interpreter that's built in to Mac OSX opens. How to install Python on Windows Unfortunately, Python doesn't come on Windows. If you're running Windows, then you need to download and install Python by following the instructions here. Installing Python on Windows isn't difficult. If you can download a file from a website, you have the skills to install Python. Fortunately, the Python Foundation (the peeps who guide the development of Python) makes installable files available from its website. Firefox and Internet Explorer responded differently to the Python download website, so the instructions are based on which of these browsers you use. If you use a whole other browser altogether, try the Internet Explorer instructions. Installing with Firefox To install Python on a Windows machine with Firefox, follow these steps: Visit www.python.org/downloads. Click the button that says Download Python 2.7.9. Or, if it's there, click a more recent version number that starts with 2.7. Clicking this button automatically downloads and saves an msi file for you. If not, try the instructions for Internet Explorer. See Figure 1. Figure 1: Download Python with Firefox. When the download's complete, click the icon for Firefox's download tool. Click the file called python-2.7.9.msi (or the more recent version, if you downloaded one). Python 2.7.9 installs on your computer. Installing with Internet Explorer To install Python on a Windows machine with Internet Explorer, follow these steps: Visit www.python.org/downloads. From the menu bar, select Downloads→Windows. You can see the menu options in Figure 2. Figure 2: Download Python with Internet Explorer. Scroll down to the heading Python 2.7.9-2014-12-10. Or scroll to a more recent version, which starts with Python 2.7, if one is available. Under this heading, click the link titled Download Windows x86 MSI Installer. See Figure 3. This is a link for a 32-bit installation, which makes things work better with third-party libraries. Use the 32-bit installer even if you have a 64-bit machine and even if you have no idea what this paragraph is talking about. Figure 3: Python x86 MSI Installer. If you're asked to choose whether to run or save the file, choose Run. This downloads python2.7.9.msi and starts running the installer. If you get a security warning when the installer begins (or at random times during the installation), choose Run. Accept the default installation options that the installer provides. How to install Python for Linux If you're running Linux, confirm that you have version 2.7.9 of Python installed, rather than version 3. This shouldn't be a problem because Python 2.7 is installed by default in recent versions of OpenSuSE, Ubuntu, and Red Hat Fedora. In the nutty odd case when someone has Python 3 but not Python 2.7, read your distribution's documentation for how to use the package manager and get Python 2.7 and IDLE.

Article / Updated 10-19-2022

While you can use Python to delete information from files, you may find you no longer need the file at all. The following steps describe how to delete files that you no longer need. Open a Python File window. You see an editor in which you can type the example code. Type the following code into the window — pressing Enter after each line: Choose Run→Run Module The application displays the File Removed! message. When you look in the directory that originally contained the ChangedFile.csv file, you see that the file is gone. The task looks simple in this case, and it is. All you need to do to remove a file is call os.remove() with the appropriate filename and path (Python defaults to the current directory, so you don’t need to specify a path if the file you want to remove is in the default directory). The ease with which you can perform this task is almost scary because it’s too easy. Putting safeguards in place is always a good idea. You may want to remove other items, so here are other functions you should know about: os.rmdir(): Removes the specified directory. The directory must be empty or Python will display an exception message. shutil.rmtree(): Removes the specified directory, all subdirectories, and all files. This function is especially dangerous because it removes everything without checking (Python assumes that you know what you’re doing). As a result, you can easily lose data using this function.