Comparing Python Command Line Parsing Libraries Argparse, Docopt, and Click. About a year ago I began a job where building command line applications was a common occurrence. At that time I had used argparse quite a bit and wanted to explore what other options were available. I found that the most popular alternatives available were click and docopt. During my exploration I also found that other than each libraries why use me section there was not much available for a complete comparison of the three libraries. Now there is this blog postThis is a guest blog post by Kyle Purdon, an Application Engineer at Bitly in Denver. If you want to, you can head directly to the source though it really wont do much good without the comparisons and step by step construction presented in this article. This article uses the following versions of the libraries 1. Python core library pip list grep click. Command Line Example. The command line application that we are creating will have the following interface python file. NAMEBasic Usage. 12. Kyle. python file. Kyle. Usage w Options Flags1. Wazzup Kyle. python file. Later Kyle. python file. Kyle. python file. Wazzup caps Kyle. This article will compare each libraries method for implementing the following features Commands hello, goodbyeArguments nameOptionsFlags greetinglt str, capsAdditional features Version Printing v versionAutomated Help Messages. Error Handling. As you would expect argparse, docopt, and click implement all of these features as any complete command line library would. This fact means that the actual implementation of these features is what we will compare. Each library takes a very different approach that lends to a very interesting comparison argparsestandard, docoptdocstrings, clickdecorators. Bonus Sections. Ive been curious about using task runner libraries like fabric and its python. I will try and put the same interface together with invoke. A few extra steps are needed when packaging command line applications, so Ill cover those steps as wellCommandLine Example. The commandline application that we are creating will have the following interface python file. NAME. Python Command Line Arguments Learn Python in simple and easy steps starting from basic to advanced concepts with examples including Python Syntax Object Oriented. You are here Home Dive Into Python 3 Difficulty level Files A nine mile walk is no joke, especially in the rain. Harry Kemelman, The. Commands. Lets begin by setting up the basic skeleton no arguments or options for each library. Argparse. 12. 34. Argument. Parsersubparsersparser. With this we now have two commands hello and goodbye and a built in help message. Notice that the help message changes when run as an option on the command hello. Docopt. 12. 34. 56. Greeter. Usage commands. Options h help Show this screen. With this we, again, have two commands hello, goodbye and a built in help message. Notice that the help message DOES NOT change when run as an option on the hello command. In addition we do not need to explicitly specify the commands. Python Print Line Number In File' title='Python Print Line Number In File' />The official home of the Python Programming Language. Python File readline Method Learn Python in simple and easy steps starting from basic to advanced concepts with examples including Python Syntax Object Oriented. Once you have written or read all of the desired data for a file object you need to close the file so that resources can be reallocated on the operating system that. Opening a file and reading the content of a file is pretty easy in Python. One easy way to read a text file and parse each line is to use the python statement. How can I get the file name and line number in python script. Exactly the file information we get from an exception traceback. In this case without raising an exception. Options section to get a help command. However, if we dont they will not show up in the output help message as options. Show this screen. Show this screen. Click. 12. 34. 56. With this we now have two commands hello, goodbye and a built in help message. Notice that the help message changes when run as an option on the hello command. Usage commands. py OPTIONS COMMAND ARGS. Show this message and exit. Usage commands. py hello OPTIONSOptions. Show this message and exit. Even at this point you can see that we have very different approaches to constructing a basic command line application. Next lets add the NAME argument, and the logic to ouput the result from each tool. Arguments. In this section we will be adding new logic to the same code shown in the previous section. Well add comments to new lines stating their purpose. Arguments aka positional arguments are required inputs to a command line application. In this case we are adding a required name argument so that the tool can greet a specific person. Argparse. To add an argument to a subcommand we use the addargument method. And in order to execute the correct logic, when a command is called, we use the setdefaults method to set a default function. Finally we execute the default function by calling args. Hello, 0. formatargs. Goodbye, 0. formatargs. Argument. Parsersubparsersparser. Kyle. python argparsearguments. Docopt. In order to add an option, we add a lt name to the docstring. The lt are used to designate a positional argument. In order to execute the correct logic we must check if the command treated as an argument is True at runtime if argumentshello, then call the correct function. Greeter. Usage basic. Options h help Show this screen. Hello, 0. Goodbye, 0. Kyle. python docoptarguments. Options. h help Show this screen. Note that the help message is not specific to the subcommand, rather it is the entire docstring for the program. Click. In order to add an argument to a click command we use the click. In this case we are just passing the argument name, but there are many more options some of which well use later. Since we are decorating the logic function with the argument we dont need to do anything to set or make a call to the correct logic. Hello, 0. formatkwargsnamegreet. Goodbye, 0. formatkwargsnameifnamemain greet1. Kyle. python clickarguments. Usage arguments. OPTIONS NAME. Show this message and exit. FlagsOptions. In this section we will again be adding new logic to the same code shown in the previous section. Well add comments to new lines stating there purpose. Options are non required inputs that can be given to alter the execution of a command line application. Flags are a boolean only TrueFalse subset of options. For example foobar will pass bar as the value for the foo option and baz if defined as a flag will pass the value of True is the option is given, or False if not. For this example we are going to add the greetinggreeting option, and the caps flag. The greeting option will have default values of Hello and Goodbye and allow the user to pass in a custom greeting. For example given greetingWazzup the tool will respond with Wazzup, name The caps flag will uppercase the entire response if given. For example given caps the tool will respond with HELLO, NAME Argparse. Argument. Parsersubparsersparser. Hello add a flag defaultFalsehelloparser. Goodbyegoodbyeparser. Wazzup Kyle. python argparseoptions. Kyle. python argparseoptions. Wazzup caps Kyle. GREETING caps name. GREETING. Docopt. Once we hit the case of adding options with defaults, we hit a snag with the basic implementation of commands in docopt. Lets continue just to illustrate the issue. Greeter. Usage basic. Smart View For Excel 2013 here. Options h help Show this screen. Uppercase the output. Greeting to use default Hello. Now, see what happens when we run the following commands 1. Kyle. python docoptoptions. Kyle. What Because we can only set a single default for the greeting option both of our Hello and Goodbye commands now respond with Hello, Kyle In order for us to make this work well need to follow the git example docopt provides. The refactored code is shown below 1. Show this screen. Say hello goodbye Say goodbyefromdocoptimportdocopt. HELLOusage basic. Show this screen. Uppercase the output. My Windows laptop had 3. I installed a single application. Installing Python 3 added almost 3,0. Files are the primary storage paradigm of every major operating system the concept is so ingrained that most people would have trouble imagining an alternative. Your computer is, metaphorically speaking, drowning in files. Before you can read from a file, you need to open it. Opening a file in Python couldnt be easier. Python has a built in open function, which takes a filename as an argument. Here the filename is exampleschinese. There are five interesting things about this filename. But that call to the open function didnt stop at the filename. Theres another argument, called encoding. Oh dear, that sounds dreadfully familiar. Bytes are bytes characters are an abstraction. A string is a sequence of Unicode characters. But a file on disk is not a sequence of Unicode characters a file on disk is a sequence of bytes. So if you read a text file from disk, how does Python convert that sequence of bytes into a sequence of characters It decodes the bytes according to a specific character encoding algorithm and returns a sequence of Unicode characters otherwise known as a string. What just happenedYou didnt specify a character encoding, so Python is forced to use the default encoding. Whats the default encodingIf you look closely at the traceback, you can see that its dying in cp. Python is using CP 1. CP 1. 25. 2 is a common encoding on computers running Microsoft Windows. The CP 1. Unicode. Decode. Error. But wait, its worse than that The default encoding is platform dependent, so this code might work on your computer if your default encoding is UTF 8, but then it will fail when you distribute it to someone else whose default encoding is different, like CP 1. If you need to get the default character encoding, import the locale module and call locale. On my Windows laptop, it returns cp. Linux box upstairs, it returns UTF8. I cant even maintain consistency in my own house Your results may be different even on Windows depending on which version of your operating system you have installed and how your regionallanguage settings are configured. This is why its so important to specify the encoding every time you open a file. So far, all we know is that Python has a built in function called open. The open function returns a stream object, which has methods and attributes for getting information about and manipulating a stream of characters. Game Sieu Quay Pha Hang Xom. After you open a file for reading, youll probably want to read from it at some point. Lets try that again. Do you see it yet The seek and tell methods always count bytes, but since you opened this file as text, the read method counts characters. Chinese characters require multiple bytes to encode in UTF 8. The English characters in the file only require one byte each, so you might be misled into thinking that the seek and read methods are counting the same thing. But thats only true for some characters. Open files consume system resources, and depending on the file mode, other programs may not be able to access them. Its important to close files as soon as youre finished with them. Well that was anticlimactic. The stream object afile still exists calling its close method doesnt destroy the object itself. But its not terribly useful. Stream objects have an explicit close method, but what happens if your code has a bug and crashes before you call close That file could theoretically stay open for much longer than necessary. Where Do You Skyrim Creation Kit. While youre debugging on your local computer, thats not a big deal. On a production server, maybe it is. Python 2 had a solution for this the try. That still works in Python 3, and you may see it in other peoples code or in older code that was ported to Python 3. But Python 2. 6 introduced a cleaner solution, which is now the preferred solution in Python 3 the with statement. This code calls open, but it never calls afile. The with statement starts a code block, like an if statement or a for loop. Inside this code block, you can use the variable afile as the stream object returned from the call to open. All the regular stream object methods are available  seek, read, whatever you need. When the with block ends, Python calls afile. Heres the kicker no matter how or when you exit the with block, Python will close that file even if you exit it via an unhandled exception. Thats right, even if your code raises an exception and your entire program comes to a screeching halt, that file will get closed. Guaranteed. In technical terms, the with statement creates a runtime context. In these examples, the stream object acts as a context manager. Python creates the stream object afile and tells it that it is entering a runtime context. When the with code block is completed, Python tells the stream object that it is exiting the runtime context, and the stream object calls its own close method. See Appendix B, Classes That Can Be Used in a with Block for details. Theres nothing file specific about the with statement its just a generic framework for creating runtime contexts and telling objects that theyre entering and exiting a runtime context. If the object in question is a stream object, then it does useful file like things like closing the file automatically. But that behavior is defined in the stream object, not in the with statement. There are lots of other ways to use context managers that have nothing to do with files. You can even create your own, as youll see later in this chapter. A line of a text file is just what you think it is  you type a few words and press ENTER, and now youre on a new line. A line of text is a sequence of characters delimited by what exactly Well, its complicated, because text files can use several different characters to mark the end of a line. Every operating system has its own convention. Some use a carriage return character, others use a line feed character, and some use both characters at the end of every line. Now breathe a sigh of relief, because Python handles line endings automatically by default. If you say, I want to read this text file one line at a time, Python will figure out which kind of line ending the text file uses and and it will all Just Work. So, how do you actually do itRead a file one line at a time, that is. Its so simple, its beautiful. You can write to files in much the same way that you read from them. First you open a file and get a stream object, then you use methods on the stream object to write data to the file, then you close the file. To open a file for writing, use the open function and specify the write mode. There are two file modes for writing. Either mode will create the file automatically if it doesnt already exist, so theres never a need for any sort of fiddly if the file doesnt exist yet, create a new empty file just so you can open it for the first time function. Just open a file and start writing. You should always close a file as soon as youre done writing to it, to release the file handle and ensure that the data is actually written to disk. As with reading data from a file, you can call the stream objects close method, or you can use the with statement and let Python close the file for you. I bet you can guess which technique I recommend.