. String Conversion (Every Class Needs a __repr__) repetition is to use the objects __class__.__name__ attribute, which will always reflect the class name as a string. var is_preview = false; Multiple decorators on a single function are applied bottom to top (decorator stacking). Covering Your A** With Assertions 2.1 Covering Your A** With Assertions Sometimes a genuinely helpful language feature gets less attention than it deserves. A Shocking Truth About String Formatting Imagine we had the following greet() function that contains an fstring: >>> def greet(name, question): return f"Hello, {name}! However, it would never catch an incorrect result: the assertion always evaluates to True, regardless of the state of the counter variable. Its relatively easy to accidentally write bad multi-line asserts due to this, well, unintuitive behavior. Then Ill apply another modification and youll see which objects are affected: >>> drect = copy.deepcopy(srect) >>> drect.topleft.x = 222 >>> drect Rectangle(Point(222, 1), Point(5, 6)) >>> rect 122 4.4. >>> q.pop(0) 'eat' collections.deque Fast & Robust Queues The deque class implements a double-ended queue that supports adding and removing elements from either end in O(1) time (nonamortized). Key Takeaways [Python Tricks, p. 171/172]: Compared to arrays, record data structures provide a fixed number of fields, where each field can have a name and may also have a different type. A Shocking Truth About String Formatting >>> user_input = '${error.__init__.__globals__[SECRET]}' >>> Template(user_input).substitute(error=err) ValueError: "Invalid placeholder in string: line 1, col 1" Which String Formatting Method Should I Use? , Python Tricks: A Buffet of Awesome Python Features. Again, were using Pythons first-class functions to pass handle_default to the get()-lookup as a fallback value. If youre willing to go beyond the Python standard library, third-party packages like NumPy14 offer a wide range of fast array implementations for scientific computing and data science. Here are a few of the best, but they still leave a lot to be desired: Stack Overflow is for asking focused, one-off questions. . Adding the missing comma after Dilbert prevents the two strings from getting merged into one: >>> names = [ 'Alice', 'Bob', 'Dilbert', 'Jane' ] But now weve come full circle and returned to the original problem. Im often amazed at some of the answers coworkers give me. . We can see that they point to identical-looking lists: >>> a [1, 2, 3] >>> b [1, 2, 3] 98 4.1. Hopefully they can do the same for you. It's fun to sort of get lost in the concepts. 6 Looping & Iteration 6.1 Writing Pythonic Loops . Decorators allow you to define reusable building blocks that can change or extend the behavior of other functions. Desktop Administrator at Tesla Inc. Hey there! You see here that we need to import the Template class from Pythons built-in string module. values = [expression for item in collection], values = [expression for item in collection if condition], hes owner and editor in chief of realpython.com, Patterns for Cleaner Python (Covering Your A** With Assertions The Zen of Python Easter Egg), Effective Functions (Pythons Functions Are First-Class Nothing to Return Here), Classes & OOP (Object Comparisons: is vs == Instance, Class, and Static Methods Demistified), Common Data Structures in Python (Dictionaries, Maps, and Hashables Priority Queues), Looping & Iteration (Writing Pythonic Loops Iterator Chains), Dictionary Tricks (Dictionary Default Values Dictionary Pretty Printing), Pythonic Productivity Techniques (Exploring Python Modules and Objects Peeking Behind the Bytecode Curtain), Caveat #1 Dont Use Asserts for Data Validation. Redemption links and eBooks cannot be resold. . If you dont remember the exact spelling for a particular class or function, then maybe thats all you need to keep going without interrupting your coding flow. . If youd like to avoid the locking overhead of queue.PriorityQueue, using the heapq module directly is also a good option. Structs are defined using a format strings-like mini language that allows you to define the arrangement of various C data types like char, int, and long, as well as their unsigned variants. . Lets introduce a custom exception type to represent a failed name validation. 281 8.2. However, I recommend that you stick with the accepted naming convention to avoid confusion. A while ago I had a discussion at work about which pattern to use for implementing a maintainable class hierarchy in Python. Exploring Python Modules and Objects a few lines at once. 297 9.2. . Copying an ob116 4.4. If you try to apply one of these decorators to a function that takes arguments, it will not work correctly. 272 7.5. First of all, the self.foo variable appears unmodified as foo in the attribute list. Adding a single underscore in front of a variable name is more like someone putting up a tiny underscore 8 cf. A short and beautiful algorithm using a queue is breadth-first search (BFS) on a tree or graph data structure. What This Book Will Do for You How to Read This Book Patterns for Cleaner Python Covering Your A** With Assertions Complacent Comma Placement Context Managers and the with Statement . 3.2 Lambdas Are Single-Expression Functions 3.3 The Power of Decorators . To use them, you need to import the collections module. Data structures. Python ships with several stack implementations that each have slightly different characteristics. . The following two decorators wrap the output string of the decorated function in HTML tags. . Cloning Objects for Fun and Profit >>> a = Point(23, 42) >>> b = copy.copy(a) If we inspect the contents of the original Point object and its (shallow) clone, we see what wed expect: >>> a Point(23, 42) >>> b Point(23, 42) >>> a is b False Heres something else to keep in mind. type: 'post', Marks a self-taught programmer and fledgling Pythonista, and hes working hard to grow his skills. For example, heres a simple dictionary. . Lets take the humble for-in loop, for example. The last thing I want is for you to pass on this book because youre not sure itll help you on your Python journey. Fields stored on classes are mutable, and new fields can be added freely, which you may or may not like. Python Tricks, A Buffet of Awesome Python Features. Namedtuples can help clean up your code by enforcing an easier-to-understand structure on your data. That way, you can just remember to always end your lines with a comma and thus avoid the comma placement juggling that would otherwise be required. Its a good idea to be careful and double-check your scoping when dealing with shared state on a class. In practical terms, this means local variables and the execution state of the generator function are only stashed away temporarily and not thrown out completely. And now Mark's dream of mastering Python is slowly withering away For example, I merrily wrote a bunch of broken test cases that gave a false sense of security in one of my test suites. If youre not looking for parallel processing support (or dont want to handle locking and unlocking manually), your choice comes down to the built-in list type or collections.deque. Cloning Objects for Fun and Profit 4.4 Cloning Objects for Fun and Profit Assignment statements in Python do not create copies of objects, they only bind names to an object. Reviewed in the United States on May 23, 2019. Consequently I had no expectations of the book at all. Imagine you had the following code in a module called my_module: # my_module.py: def external_func(): return 23 37 2.4. The downside here is that the AlwaysBlueCar constructor now has a rather unhelpful signaturewe dont know what arguments it expects without looking up the parent class. . It pays off to read up on this string formatting mini-language in the Python documentation.13 In Python 3, this new style string formatting is preferred over %-style formatting. If theyre equal, the next two items at index one are compared, and so on. CPython mailing list 158 5.1. You can use json.dumps() to prettyprint Python dicts with nicer formatting: >>> import json >>> json.dumps(mapping, indent=4, sort_keys=True) { "a": 23, "b": 42, "c": 12648430 } These settings result in a nicely indented string representation that also normalizes the order of the dictionary keys for better legibility. And, they let you do that without permanently modifying the wrapped function itself. dict Your Go-To Dictionary Because of their importance, Python features a robust dictionary implementation thats built directly into the core language: the dict 156 5.1. Cloning Objects for Fun and Profit Rectangle(Point(999, 1), Point(5, 6)) >>> srect Rectangle(Point(999, 1), Point(5, 6)) Voila! >>> greet('Guido') 'Hello, Guido!' While the insertion point can be found in O(log n) time using bisect.insort40 in the standard library, this is always dominated by the slow insertion step. This is a core Python feature but your code might communicate its intent more clearly with an explicit return None statement. There you typically also want to accept arbitrary arguments to be passed through to the wrapped function. Free Weekly Tips for Python Developers 9.1 Free Weekly Tips for Python Developers Are you looking for a weekly dose of Python development tips to improve your productivity and streamline your workflows? In this chapter Im going to give you an introduction to using assertions in Python. Learning the ins and outs of Python is difficultand with this book you'll be able to focus on the practical skills that really matter. Compared to the standard string representation for dictionaries, what we get here is much easier on the eyes. Generators Are Simplified Iterators 'Hi' 'Hi' 'Hi' Great! . So lets talk a little more about that. Key Takeaways [Python Tricks, p. 237/238]: Key Takeaways [Python Tricks, p. 257/258]: Software Developer for rapid prototype or high quality software with interest in distributed systems and high performance on premise server applications. Yet another use case for the sushi operator is creating (shallow) copies of existing lists: >>> copied_lst = lst[:] >>> copied_lst [7, 8, 9] >>> copied_lst is lst False Creating a shallow copy means that only the structure of the elements is copied, not the elements themselves. . Now we have a generator that stops producing values after a configurable number of repetitions. It raises a StopIteration exception to signal weve exhausted all of the available values in the iterator. Farooq A and Zaytsev V There is more than one way to zen your Python Proceedings of the 14th ACM SIGPLAN International Conference on Software Language Engineering, (68-82). *args and **kwargs let you write functions with a variable number of arguments in Python. Ben Felder, Pythonista. When it comes to memory usage, they are also better than regular classes and just as memory efficient as regular tuples: 20 cf. If we call the function with additional arguments, args will collect extra positional arguments as a tuple because the parameter name has a * prefix. return 'Hello!' However, with self.__baz things look a little different. Python Docs: collections.deque cf. Python Docs: heapq Priority queue implementation notes 43 cf. Beautiful Iterators Now, what does this for-in loop really do behind the scenes? They can be used whenever function objects are required. >>> a == c True 99 4.1. This should help you apply the pattern in other situations as well. Heres an example: >>> import pprint >>> pprint.pprint(mapping) {'a': 23, 'b': 42, 'c': 12648430, 'd': set([1, 2, 3])} You can see that pprint is able to print data types like sets, and it also prints the dictionary keys in a reproducible order. Generators abstract away much 237 6.5. Namespaces are one honking great idea lets do more of those! . In this case, the lambda function provides a quick way to modify the sort order. Printing dictionaries with the json module only works with dicts that con274 7.6. Python Tricks: A Buffet of Awesome Python Features 74 3.3. These specialized dictionaries are all based on the built-in dictionary class (and share its performance characteristics), but add some convenience features on top of that. The resulting string is intended more as a debugging aid for developers. This type of method doesnt take a self or a cls parameter, although, of course, it can be made to accept an arbitrary number of other parameters. Reading up on virtual machine theory is enlightening (and a ton of fun). Heres how to create a custom exception hierarchy for all exceptions in a module or package. Pro tip: If you ran the last example inside a Python REPL session or from the terminal, and you want to stop it, hit Ctrl + C a few times to break out of the infinite loop. Pythons sets are backed by the dict data type and share the same performance characteristics. This accumulates their effects and its what makes decorators so helpful as reusable building blocks. 153 Alright, so data structures in Python, eh? Many times both of these responsibilities can be shouldered by a single class. . Array Data Structures bytearray Mutable Arrays of Single Bytes The bytearray type is a mutable sequence of integers in the range 0 > arr = bytearray((0, 1, 2, 3)) >>> arr[1] 1 # The bytearray repr: >>> arr bytearray(b'x00x01x02x03') # Bytearrays are mutable: >>> arr[1] = 23 >>> arr bytearray(b'x00x17x02x03') >>> arr[1] 23 # Bytearrays can grow and shrink in size: >>> del arr[1] >>> arr bytearray(b'x00x02x03') >>> arr.append(42) >>> arr bytearray(b'x00x02x03*') 13 cf. By the way, OrderedDict is not a built-in part of the core language and must be imported from the collections module in the standard library. Complacent Comma Placement 2.2 Complacent Comma Placement Heres a handy tip for when youre adding and removing items from a list, dict, or set constant in Python: Just end all of your lines with a comma. . Its a great skill to learn that will help bring your Python knowledge to the next level and make you a more well-rounded Pythonista. If the container is ordered, the resulting sequence of elements will be too. Asserts can be globally disabled with an interpreter setting. Im sure youve been wondering why this printf-style formatting is called old style string formatting. And the best part - you can use it as go-to table book with hints! All of this is completely transparent to the programmer. Python is differentan effective developer will spend quite a bit of time in REPL sessions working interactively with the Python interpreter. Wikipedia Total order 201 5.7. Just like any other tool in the software development toolbox, decorators are not a cure-all and they should not be overused. Some developers are surprised when they learn that its possible to call a static method on an object instance. Wouldnt it be much nicer if we could just explode a vector object into its three components and pass everything to the print_vector function all at once? Heres a greet function that formats a greeting string using the function object passed to it and then prints it: def greet(func): greeting = func('Hi, I am a Python program') print(greeting) You can influence the resulting greeting by passing in different functions. Heres a slightly more complex decorator which converts the result of the decorated function to uppercase letters: 76 3.3. Use __unicode__ instead of __str__ in Python 2. Python Tricks started out as a short series of Python code screenshots that I shared on Twitter for a week . Avoid this naming scheme for your own attributes. Python favors a simpler and more human naming scheme, and I love it. You can copy arbitrary objects (including custom classes) with the copy module. . Python Docs: collections.Counter 187 5.4. Execution can be resumed at any time by calling next() on the generator: >>> iterator = repeater('Hi') >>> next(iterator) 'Hi' 233 6.5. But from these inquiries I got the sense that my short-and-sweet code examples would be worth exploring as a teaching tool. Class vs Instance Variable Pitfalls class Dog: num_legs = 4 # > jack = Dog('Jack') >>> jill = Dog('Jill') >>> jack.name, jill.name ('Jack', 'Jill') Theres a little more flexibility when it comes to class variables. Python Docs: String literal concatenation 26 2.2. Getting a raw listing of the attributes on an object wont always be enough information to solve the problem at hand. I chose not to do this with the first example in this chapter because it mixes up the cleanliness of the mental model behind the iterator protocol. 14 BUILD_STRING 5 16 RETURN_VALUE 14 cf. Priority Queues Whenever I get stuck in python (usually with flask blueprints or I feel like my code could be more pythonic) I post questions in our internal Python chat room. I believe that going back to the fundamentals always pays off for a programmer, regardless of their skill level or experience. . . Template strings are not a core language feature but theyre supplied by a module in the standard library. 258 7.3. Instance, Class, and Static Methods Demystified This confirms that static methods can neither access the object instance state nor the class state. What This Book Will Do for You How to Read This BookPatterns for Cleaner Python Covering Your A** With Assertions Complacent Comma Placement Context Managers and the with Statement Underscores, Dunders, and More A Shocking Truth About String Formatting The Zen of Python Easter EggEffective Functions Python's Functions Are First-Class Lambdas Are Single-Expression Functions The Power of Decorators Fun With *args and **kwargs Function Argument Unpacking Nothing to Return HereClasses & OOP Object Comparisons: is vs == String Conversion (Every Class Needs a __repr__) Defining Your Own Exception Classes Cloning Objects for Fun and Profit Abstract Base Classes Keep Inheritance in Check What Namedtuples Are Good For Class vs Instance Variable Pitfalls Instance, Class, and Static Methods DemystifiedCommon Data Structures in Python Dictionaries, Maps, and Hashtables Array Data Structures Records, Structs, and Data Transfer Objects Sets and Multisets Stacks (LIFOs) Queues (FIFOs) Priority QueuesLooping & Iteration Writing Pythonic Loops Comprehending Comprehensions List Slicing Tricks and the Sushi Operator Beautiful Iterators Generators Are Simplified Iterators Generator Expressions Iterator ChainsDictionary Tricks Dictionary Default Values Sorting Dictionaries for Fun and Profit Emulating Switch/Case Statements With Dicts The Craziest Dict Expression in the West So Many Ways to Merge Dictionaries Dictionary Pretty-PrintingPythonic Productivity Techniques Exploring Python Modules and Objects Isolating Project Dependencies With Virtualenv Peeking Behind the Bytecode CurtainClosing Thoughts Free Weekly Tips for Python Developers PythonistaCafe: A Community for Python Developers, Python Tricks: The Book Dan Bader For online information and ordering of this and other books by Dan Bader, please visit dbader.org. In practical terms, this means not only can functions return behaviors but they can also pre-configure those behaviors. You see, when a return statement is invoked inside a function, it permanently passes control back to the caller of the function. This turns any assert statement into a null-operation: the assertions simply get compiled away and wont be evaluated, which means that none of the conditional expressions will be executed. . This was a great book to binge after attempting to read the wall-of-text that is Fluent Python. Everything is about the facts, not the people. . Lets redefine our car class so it contains both tostring dunder methods with outputs that are easy to distinguish: 103 4.2. . You need immutable fields: In this case, plain tuples, collections.namedtuple, and typing.NamedTuple would all make good options for implementing this type of data object. You can use the regular @ syntax for that and just stack multiple decorators on top of a single function: @strong @emphasis 79 3.3. Well start by creating an instance of the class and then calling the three different methods on it. I had no pressure to get better in Python. 4.8 Instance, Class, and Static Methods Demystified . Beautiful Iterators 6.4 Beautiful Iterators I love how beautiful and clear Pythons syntax is compared to many other programming languages. Lambda functions behave just like regular functions declared with the def keyword. Python dictionaries are backed by a hash table data structure. And as a result, the whole structure takes up more space. . It is not an entry-level Python course. A good way to view them is to think that namedtuples are a memoryefficient shortcut to defining an immutable class in Python manually. Modifying the global environment often requires you to run the pip install command with superuser (root/admin) credentials. , youll pick up the idioms and practical tips you need to become a confident and effective Pythonista. Emulating the same thing with a Python list would be impossibletheres no way we could create a list with an infinite number of elements in the first place. He explains important Python aspects with clear examples (using two twin cats to explain is vs == for example). First, the input function was wrapped by the @emphasis decorator, and then the resulting (decorated) function got wrapped again by the @strong decorator. Behold the beauty of the for-in loop! And voila, we just traced back how our greet() function gets executed internally by the CPython virtual machine. utilizing Python's Virtualenv library. Unsere Partner sammeln Daten und verwenden Cookies zur Personalisierung und Messung von Anzeigen. . If you need a list object right away, youd normally just write a list comprehension from the get-go. Find out more about Dan at www.dbader.org. . Unicode is the preferred and future-proof way of handling text in your Python programs. So without further ado, here is the code snippet. The approach and the substance were aimed at someone who is already familiar with the basics of Python and programming, but wanted to improve those skills in a non-formal and unsystematic way. Saving a few keystrokes wont matter in the long run, but your colleagues (and your future self) will appreciate clean and readable code more than terse wizardry. So youll get a more or less random order when you add items to a set container.
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