Announcing PythonDAB: The course I’ve always wanted to teach

I spend each day teaching Python and data science to companies around the world. From “Python for non-programmers” to day-long advanced workshops, I earn my living by helping people to improve their skills — which is great for their own careers, and great for the organizations where they currently work.

Reuven Lerner's Python Data Analytics Bootcamp

However much I love teaching these classes, I often feel like I’m not able to teach the way that I want. I’d like to have more exercises. More collaboration. More time for the ideas to sink in. More time for people to ask questions, and understand the ideas behind the technologies that I’m teaching.

This is especially frustrating to me, someone with a background — a PhD, even! — in the field of education. I have strong ideas about how people can and should learn, and while I do my best to incorporate these ideas into my corporate training, the inherent nature of such classes is somewhat limiting.

That’s why I’m so excited to announce my biggest and most ambitious course ever: PythonDAB, my Python Data Analytics Bootcamp. This course combines everything I know about Python and data analytics, along with everything I know about learning, with input from industry leaders who told me what they are looking for in entry-level data analysts.

A few salient details about PythonDAB:

  • You’ll learn Python from the ground up — not just the syntax, but also how the language works, and why it works the way it does. This includes such topics as comprehensions and object-oriented programming.
  • You’ll learn Git, and how to collaborate on GitHub.
  • There will be numerous exercises, and you’ll share them with the rest of the group. Moreover, you’ll provide feedback to other people on their code, and they’ll provide it on yours. Learning to review other people’s code, and to accept criticism, is an important part of becoming a Python developer.
  • You’ll learn NumPy, and then Pandas, the two libraries used for data analytics in the Python world. We’ll talk about everything from loading data, to cleaning it, to optimizing the size of your data, to visualization, to producing output in a variety of formats.
  • You’ll analyze real-world data sets, working with others in your cohort to answer questions about our everyday world.
  • We’ll have a private forum, in which you can ask questions and generally share ideas and insights with others in the cohort.
  • Finally, I’ll be an active part of this class, with 1-2 sessions of office hours each week — sometimes for Q&A, sometimes to give you a deeper understanding of a topic, and sometimes to explore new things that I’ve discovered myself, and want to share.

This isn’t like any other course you’ve ever seen — and it’s certainly not like any online course you’ve ever bought, in which the videos sit on your disk, waiting to (maybe, perhaps, some day) be viewed when you have a chance. This is an active class, with a live instructor, who wants to see you learn and improve.

PythonDAB starts on June 1st, and goes for 16 weeks, ending on September 15th, and is designed to take about 10 hours/week of your time.

  • If you’re relatively new to Python, and want to get ahead in the world of data, then this course is for you.
  • If you have some experience with data, and want to learn Python to get a better job, then this course is for you.
  • If you’re looking to improve your current job, or switch to a new career in data analytics, then this course is for you.

But before you can sign up, you should really learn many more details. And that’s what I’ll be providing at a webinar this Wednesday, May 25th, at 10 a.m. Eastern. Sign up for the webinar at https://PythonDAB.com/, and I’ll not only present more details, but answer all of your questions.

I’ve waited a long time to be able to offer this kind of class. I hope that you’ll want to join me in leveling up your skills.

The “why” of object-oriented programming (in Python)

When I ask people — in my corporate training, or when they join my “Better developers” newsletter — what frustrations they have with Python, many people say that it’s object-oriented programming. Even if they have a decent background in procedural (i.e., traditional, non-OO) development, objects confuse and surprise them.

That’s a shame, because the whole point of objects is to help us organize our code better, to make it easier to both develop and maintain programs.

A lot of introductions to objects focus on the “how,” talking about syntax — creating classes, defining methods, and the like. But I haven’t seen a lot of discussions of the “why,” or comparing how you would accomplish something without objects vs. how you would do so with them.

In this short (15-minute) video, I provide just this sort of introduction, using Python as the language. I first show how I can use tuples and functions to accomplish a simple task, and then show how you can use a class and methods to do the same thing. After watching this video, I hope that you’ll have a better sense of why and when we can use objects, and what advantages they can give us.

Questions or comments? Let me know here, on Twitter (@reuvenmlerner), or even via e-mail, at reuven@lerner.co.il.

Save time — and 30% — with my 25 Python, Git, and data science courses!

Reuven Lerner, Python Training

It’s that time of year — Black Friday, when everything goes on sale. At my online store, with more than 25 courses about Python, data science, Git, and regular expressions, I’m giving 30% off of the usual price.

Here’s what some students have said about my courses:

  • “This is by far the most amazing Python course I have taken. It is full of advanced information, especially for people who want to take their Python skills to next level.” — Punit Jain
  • “I think you have an exceedingly rare crystal clear explanation of all things Python and I look forward to more content!” — David Punsalan
  • “Reuven’s teaching approach is excellent in terms of pacing and cadence. I never felt that too much was being covered at one time or that a topic was covered too fast before moving onto to the next item.” — Jason Wattier

Every single one of my courses also includes exercises — not only questions and answers, but a walk-through of the solution, live-coding into Jupyter notebooks.

No matter how long you’ve been using Python, one of my courses will help you to write better, clearer, more idiomatic, more efficient code. And you’ll do it in less time, freeing you to do other things, such as spend time with family, or relax with friends.

Here’s what you can find in my online store, at https://store.lerner.co.il/:

And yes, in case you’re wondering: I’ve added a bunch of courses in the last year, including intro courses about Python files and modules, and advanced courses (recordings of Webinars) about functional Python, packages, threading, and asyncio.

Just use the coupon code BF2021 at checkout to get 30% off. It’s a bit hard to see where you can enter the coupon code, so I’ve included it in the above links.

Wondering what course(s) are best for you? E-mail me at reuven@lerner.co.il, or DM me on Twitter at @reuvenmlerner.

Stop fearing Python decorators

If I mention “decorators,” even if you’ve been programming with Python for a while, what comes to mind?

To many, decorators are mysterious and powerful — and beyond the reach of mere mortals. Their @ signs pop up in all sorts of places, from “pytest” to Flask to properties… but it’s not really clear what they are, what they’re doing, how they work, or how they can be useful.

Here’s the thing: If you are comfortable writing functions and classes in Python, then you can write decorators. And once you start, you’ll wonder how you lived without them.

I’ve taught decorators to engineers at some of the world’s largest companies, and also at conferences like PyCon and Euro Python. And this Sunday, I’ll be teaching a live, online course all about decorators.

Once you’re done with the course, you’ll be able to write and understand a wide variety of decorators, from those that filter arguments to those that cache function results and monitor your code’s performance.

I’m running these class like I do my corporate training: With lots of time for Q&A, and a ton of hands-on exercises. Without slides, as I live-code into Jupyter. We’ll even have a private forum, so you can ask questions when the class is done. And of course, you’ll have access to the video recording.

If you want to finally understand decorators, then you want this class.  Sign up here: https://store.lerner.co.il/live-class-decorators

Questions? Comments? Eligible for a discount (for students, retirees/pensioners, or people living outside the world’s 30 richest countries)? E-mail me at reuven@lerner.co.il or get me on Twitter as @reuvenmlerner, and I’ll answer ASAP.

Let’s de-confuse Python objects!

Are you confused about object-oriented Python? You’re not alone. In teaching Python to companies around the world for more than 20 years, I’ve found that almost everyone is confused by Python objects:

  • Newcomers to object-oriented programming are just plain ol’ confused — by the terminology, the syntax, and why we even need to use objects.
  • People with experience using objects in other language wonder why Python’s objects are so weird. Why do we need to use “self” so much? And what’s with the static fields that behave strangely?
  • Even people with Python experience, who have been using objects for a while, find the object system to be a confusing mish-mash of seemingly consistent rules.

Sounds like you? Well, I have good news: Starting this Sunday, I’ll help you make sense of it all in live, online classes about object-oriented Python.

When you’re done, you’ll not only have a clearer mental model. You’ll also be able to write shorter, more expressive, more idiomatic, more maintainable code.

In other words, you’ll be able to do more in less time, and with less code. That’ll help you in your current job — and can improve your chances of getting a new, better job.

So, what am I teaching?

September 12th: Intro Python Objects

  • Classes
  • Instances
  • Attributes
  • Class attributes
  • Inheritance
  • Basic magic methods

September 19th: Advanced Python Objects

  • Advanced magic methods
  • Context managers
  • Multiple inheritance
  • Metaclasses
  • Properties
  • Descriptors

I’m running these classes like I do my corporate training: With lots of time for Q&A, and a ton of hands-on exercises. Without slides, as I live-code into Jupyter. We’ll even have a private forum, so you can ask questions when the class is done. And of course, you’ll have access to the video recording.

If you’re confused about Python objects, or want to take your understanding to the next level, then these classes are for you.

Questions? Comments? Eligible for a discount (for students, retirees/pensioners, or people living outside the world’s 30 richest countries)? Hit me up on Twitter (@reuvenmlerner) or e-mail (reuven@lerner.co.il), and I’ll answer ASAP.


PS: These are two of the six live courses I’m running in September and October. Take a look at all six topics here: https://store.lerner.co.il/liveclasses

Missing my blog posts? Check me out on YouTube!

I haven’t been posting much to this blog of late, in part because I’ve been posting Python-related videos on YouTube. Just in the last month or two, I’ve posted:

I plan to write more on this blog as well, but don’t forget to check out my YouTube channel for these Python lessons. And all of the Jupyter notebooks I use in that channel are available on GitHub for you to download, at https://github.com/reuven/youTube-notebooks.

If you have topics you want me to explore, just e-mail me at reuven@lerner.co.il, or DM me on Twitter at @reuvenmlerner.

Level up your Python skills with six new courses

Want to level up your Python skills, to solve more problems in less time, and with less code?

Want to take advantage of the latest techniques in the Python world?

Want to ensure that your code is as idiomatic as possible, so that whoever has to maintain it down the line (including you) will have an easier time working with it?

Well, good news: In September and October, I’ll be teaching six different live Python courses that will help you to achieve those goals:

  • September 12th: Intro Python objects — a live version of one of my most popular recorded and corporate-training courses, introducing object-oriented programming in Python. We’ll talk about (among other things) classes, instances, attributes, and inheritance.
  • September 19th: Advanced Python objects — again, a live version of a popular recorded class, showing how objects work behind the scenes and some advanced techniques for working with them, including such topics as magic methods, properties, and descriptors.
  • September 26th: Functional programming in Python — Learn about various functional techniques in Python, including partial functions, dispatch tables, and such functions as map, filter, and reduce.
  • October 3rd: Threading and multiprocessing —  Want to know how to use threads and processes to add concurrency to your programs? We’ll discuss both of these techniques, including their pros and cons.
  • October 10th: asyncio — The newest form of Python concurrency, ascynio, is different from threading and multiprocessing. Learn what it is, how to use it, and how to write clients and servers using asyncio.
  • October 17th: Creating a Python project — Your goal in writing Python code isn’t just to write the code, but rather to create something that you can distribute to others, either as a library or application. In this class, we’ll discuss the structure of a project, and how to distribute it.

Want to buy access to all six courses?  You can do this with this course bundle — which also gives you a 15% discount, or the ability to pay in installments: https://store.lerner.co.il/bundle-six-live-courses

Several additional points:

  • This is the same sort of class that I’ve taught on-site for Fortune 500 companies for 20 years (and online to many of them since the pandemic started), full of examples, clear explanations, and bad jokes.
  • You’ll have a chance to ask me questions while they’re running.
  • I’m going to set up a forum for each class, so that if you have questions once the class is done, you’ll still be able to ask me, and/or be in touch with other participants.
  • Every class will include numerous hands-on exercises.
  • Each class will start at 6 p.m. in Israel, which is 11 a.m. Eastern, and will go for about 4 hours.
  • Recordings will be available within about 24 hours of the class taking place.  You’ll have access to that recording forever.
  • You’ll also have access to any files, Jupyter notebooks, and programs I produce while teaching the class.
  • As always, I offer discounts to students, seniors/pensioners, people permanently living outside of the world’s 30 richest countries, and also anyone who has been affected by the covid/coronavirus pandemic. Just reply to this message, and I’ll give you details.

Questions or comments? Just contact me at reuven@lerner.co.il or as @reuvenmlerner on Twitter. I’ll be happy to answer.

To learn more about each class, and to sign up, check out the “live classes” section of my online store: https://store.lerner.co.il/live-classes.

I look forward to seeing you in one or more of these classes!

Improve your “pandas” skills with “Pandas Workout”

As you might know, I’ve been working lately on my new book, Pandas Workout, with 50 exercises to improve your fluency using the “pandas” library for data analysis.

Well, I have exciting news: The book is now available as a MEAP, aka the “Manning Early Access Program.” This means that if you buy the book now, you’ll have access to the chapters as they’re written.

The text needs further editing, as well as technical reviews, graphics, diagrams, indexing, and the like. So it’s not done. But it’s definitely in good enough shape to start improving your “pandas” chops, allowing you to understand how the library works and how to use it better.

Also: The “50 exercises” description is a bit incorrect: Every exercise has three “Beyond the exercise” problems, taking the existing question to the next level or asking you to perform a related task. So when I say “50 exercises,” I really mean, “200 exercises.” But hey, what’s a factor of 4 among friends?

Even better: I’ve created Jupyter notebooks for all of the exercise solutions, including the “beyond” exercises — so you can download and experiment with those on your own computer.

But wait, there’s more!

To celebrate the release of this MEAP, Manning has provided a 50% discount. Just use the coupon code “mllerner250” at checkout.

You can get a copy of Pandas Workout at https://PandasWorkout.com/.

And hey, while you’re at it — you can also check out Python Workout, at https://PythonWorkout.com/.

Questions? Just e-mail me at reuven@lerner.co.il, or hit me up on Twitter as @reuvemmlerner.

And now? It’s time to get back to writing…


Python parameters primer

Python makes it easy to write functions. For example, I can write:

def hello(name):
    return f'Hello, {name}!'

I can then run the function with:


which will then, not surprisingly, return the string

'Hello, world'

Here’s a question that doesn’t come up much: How does Python assign the string argument ‘world’ to the parameter “name”? That is, how does Python know that the value “world” should be assigned to the variable “name”?

You might be thinking, “It’s obvious. I mean, there’s only one value, and there’s only one variable. How else could it work?”

But of course, programming languages don’t work based on what we think is obvious. They have rules, and follow those rules to the letter. So, what are the rules for assigning arguments to parameters?

It turns out that this question isn’t as trivial as it seems. And that’s because while function parameters in Python don’t indicate what types of values they can accept, they do tell Python what types of arguments they can accept — that is, the ways in which they can (or cannot) be assigned arguments when the function is called. (And yes, type hints sorta kinda let us indicate what types of values can be assigned to parameters, but the Python language itself ignores these.)

In this blog post, I want to review the seven (!) different types of parameters, and how Python assigns arguments to them.

Positional vs. keyword arguments

Before we can talk about parameters, we have to talk about arguments. And yes, most developers will confuse these two terms, or use them interchangeably. Simply put, an argument is a value that is passed to a function, whereas a parameter is a variable to which we assign a value. Parameters are local variables, and work just like all other local variables in Python.

When you call a function, you can pass values as two different types of arguments:

  • Positional arguments, where the position of the argument in the call indicates which parameter should get its value
  • Keyword arguments, which always have the form name=value, tell the receiving function which parameter (name) should be assigned the argument (value).

That’s it: There are no other types of arguments in Python! Everything is either positional or keyword. Parameters can indicate how they’re willing to be assigned arguments.

Simple parameters

The simplest, and most recognizable, parameters in Python are the ones without any special decorations. For example, the “hello” function from the top of this blog post has a single, simple parameter:

def hello(name):
return f'Hello, {name}!'

I can call this function with a single positional argument:


Or I can call this function with a single keyword argument:


In the first case, Python sees that we have passed a single positional argument. It checks the function’s definition to see how many arguments we will accept (__code__.co_argcount in the function object) and then grabs the parameter’s name (__code__.co_varnames in the code object). Python thus knows that argument #1 should be assigned to parameter #1 — meaning, “world” should be assigned to “name”.

In the second case, Python sees that we have passed a single keyword argument, with a name “name” and a value “world”. It checks (again, with __code__.co_argcount and __code__.co_varnames) whether there is a parameter called “name”. Once it discovers that there is, it assigns “world” to the variable “name”, and all is good.

What happens if we pass the wrong number of positional arguments? Python notices this, and raises an exception, telling us how many arguments the function expects to get, and how many we actually passed. (We’ll get back to this in a little bit.) For example:

>>> hello('a', 'b', 'c')
Traceback (most recent call last):
File "", line 1, in 
TypeError: hello() takes 1 positional argument but 3 were given         

What happens if we pass keyword arguments for parameters that don’t exist? Python notices this as well, and raises an exception. For example:

>>> hello(a=10, b=20, c=30)
Traceback (most recent call last):
File "", line 1, in 
TypeError: hello() got an unexpected keyword argument 'a'

Multiple parameters

What if a function has multiple parameters? Then we can invoke it with positional arguments, keyword arguments, or even a mixture of these. For example, let’s take the following function:

def calc(first, op, second):
   if op == '+':
       return first + second
   elif op == '-':
       return first - second
       return f'Operator {op} is unsupported'

I can call this function in a variety of ways:

  • Only with positional arguments, as in “calc(10, ‘+’, 3)”
  • Only with keyword arguments, as in “calc(first=10, op=’+’, second=3)”
  • Keyword arguments in a different order, as in “calc(second=3, first=10, op=’+’)”.
  • A mixture of positional and keyword arguments, as in “calc(10, op=’+’, second=3)”

This last example is perhaps the most interesting of the bunch. That’s because Python always allows you to mix positional and keyword arguments, so long as all of the positional arguments come before all of the keyword arguments. Here’s what happens if you get this wrong:

>>> calc(first=10, op='+', 3)
File "", line 1
calc(first=10, op='+', 3)
SyntaxError: positional argument follows keyword argument

If a function has these sorts of simple parameters, then we can call the function with any combination of positional and keyword arguments, so long as the number of arguments matches the number of parameters and all of the positional arguments come before the keyword arguments.

Parameters with defaults

Sometimes, we want to give a parameter a default value, so that the caller doesn’t need to provide an argument. We can do this with a syntax that’s identical to that of keyword arguments, name=value, except that this is done in the function definition, not in the function call. For example, we can rewrite our “calc” function as follows:

def calc(first, op='+', second=0):
   if op == '+':
       return first + second
   elif op == '-':
       return first - second
       print(f'Operator {op} is unsupported')

The above function is identical to the previous version, except that we’ve now indicated that the caller is only required to pass one argument, which will be assigned to the parameter “first”. Both “op” and “second” are now optional, because if we don’t pass them any arguments, they’ll get the default values “+” and 0.

This is possible because when we create our function, Python creates an attribute, “__defaults__”, on the newly created function object. __defaults__ is a tuple containing all of the default values. When we call the function, it checks __code__.co_argcount to find out how many arguments we’re supposed to pass — and if we’re missing one or more, Python grabs the missing one(s) from __defaults__.

In order to make this work, parameters with defaults must be defined after those that lack defaults. That makes it possible for Python to figure out which parameters didn’t get values from the caller, and thus need to get values from __defaults__.

For example, __defaults__ in the “calc” function is (‘+’, 0). So if I invoke “calc(5)”, Python sees that we need to pass three arguments. But we’ve only passed one, so it needs to retrieve the final two values from calc.__defaults__ and then assign those values to “op” and “second”. After it does that, it invokes the function — whose body is blissfully unaware of how this happened.

It’s important to point out that the default values are set when the function is defined, and thus compiled. For this reason, you can get into big, big trouble if you use a mutable default, such as a list. For example:

def add_one(x=[]):
    return x

You might think that the default value in the above function means: If I call “add_one” without any arguments, then x will be assigned to an empty list. But no! The list is created when we define the function, and stored away in __defaults__. This means that every time we call the function without any arguments, Python will grab the list from __defaults__ and assign it to x. The function will modify the list, which not only affects x, but also affects the list as stored in the __defaults__ tuple. In other words, our empty list will not be very empty after several calls to the function.

The moral of this story is: Never use mutable default values in a function. Code checkers like “pylint” will notice this for you, but once you realize just how bad this can be, you will likely avoid it without any help.

So far, both of these parameters types can be assigned arguments via either positional or keyword arguments. Now let’s shake things up a bit, though, and talk about the weirder and more unusual parameter types.

*args (aka: all unclaimed positional arguments)

Many people are familiar with the term “*args”, and even use it a bit — but I’ve found that they often don’t understand just what it’s for, or how to use it.

If you have a parameter whose name is preceded by “*”, then its type will be a tuple, and it will contain all of the positional arguments that weren’t assigned to another parameter. Traditionally, this parameter is called “*args”, and pronounced “splat-args,” on the assumption that the “*” sign looks something like a squashed bug. You can call this parameter whatever you want, though — and I often do.

Once your function has *args as a parameter, there is no maximum number of positional arguments that it can take. Any extras, as it were, will be placed into *args. For that reason, *args must be the last parameter that can take positional arguments. (We’ll see some others which can only take keyword arguments in a little bit.) If there are any parameters preceding *args, then *args will be assigned those arguments that weren’t grabbed by the other parameters.

I should note that there isn’t really a technical advantage to use *args. You could, of course, just have a single parameter that takes a list or tuple. But there’s something that feels natural about passing a bunch of values as individual arguments, rather than passing a list or tuple. For example, if I have a function that sums numbers, then it might make more sense to pass he numbers as separate arguments than to pass a list. If I have a function that performs a network security check on IP addresses, then I could take the addresses as positional arguments, rather than as a list. And if I have a function that summarizes data from files, it would make sense to have the function take filenames (strings), rather than a list of filenames.

For example, we can write a number-summing function as follows:

def mysum(numbers):
    total = 0
    for one_number in numbers:
        total += one_number
    return total

To run the above function, I need to pass an iterable of numbers, as in “mysum([10, 20, 30])”. That’s because the function, as defined, takes a single argument (either positional or keyword). But if I change the function definition ever so slightly:

def mysum(*numbers):
    total = 0
    for one_number in numbers:
        total += one_number
    return total

Now the function expects to get only positional arguments, and any number of them. The parameter (i.e., variable) “numbers” will now be a tuple, containing whatever values were passed to the function. I can now call:

  • mysum(10, 20, 30)
  • mysum(1,2,3,4,5)
  • mysum() # no elements, returns 0
  • mysum(*[10, 20 ,30]) # a bit advanced — turns a list into individual arguments!

What if the function has other parameters that accept positional arguments? Those must be before *args. And we can have as many of those as we like. For example:

def myfunc(a, b, *args):
    return f'{a=}, {b=}, {args=}'

If I call “myfunc(10, 20, 30, 40, 50)”, then we’ll see that a is 10, b is 20, and args is the tuple (30, 40, 50).

The typical use for *args is in a “for” loop, list comprehension, or similar iteration. If your function accepts *args but then pulls out index 0, 1, or 2 specifically, then you are probably using it wrong. Similarly, you probably don’t want to be calling “len” on the tuple. The idea is that you’ll do the same thing with all of the arguments, no matter how many (or few) there might be.

What happens if I want my function to have a parameter with a default, and then take *args? For example:

def myfunc(a, b=10, *args):
    return f'{a=}, {b=}, {args=}'

If I run “myfunc(5,6,7,8,9)”, then it’s pretty obvious that a will be 5, b will be 6, and args will be the tuple (7,8,9). But what if I want to pass a value for “a”, pass additional positional arguments to “args”, and have the default value of “b” in place?

Answer: You can’t. There isn’t any way to do that. Because along the way to *args, you have to give “b” a value. (We’ll see in a bit how we can use keyword-only parameter to accomplish this.)

You might be thinking: Of course there’s a way to do this! I can just call the function, passing “args” as a keyword argument. Then I can sidestep “b” and give a value to “args”. But that won’t work, because you would be assigning to “args” as a keyword argument, and args (by definition) only takes positional values. You cannot use “args” as the name in a keyword argument; Python will tell you that it’s not aware of such a parameter.

**kwargs (aka: all unclaimed keyword arguments)

Just as *args allows a function to accept as many positional arguments as we might want to pass, **kwargs allows a function to accept as many keyword arguments as we might like. As with *args, the name “kwargs” is traditional but not mandatory; you can use any name you want. The variable “kwargs” is a dict in which the keys are strings, and the values are whatever was passed in the keyword arguments For example:

def myfunc(a, b=10, **kwargs):
   return f'{a=}, {b=}, {kwargs=}'

If I call “myfunc(5, 6, 7, 8)”, then I’ll get an error — because the function only accepts up to two positional arguments. All the rest must be keyword arguments! So I can try “myfunc(5,6, x=100, y=200)” and then we’ll see the following output:

"a=5, b=6, kwargs={'x': 100, 'y': 200}"

Notice that our keyword arguments (x and y) were turned into strings. Both 100 and 200 remained as integers, as they were passed.

Also notice that we don’t have the problem with “b” that we did before, with *args. We can just call “myfunc(5, x=100, y=200)”, and we’ll get:

"a=5, b=10, kwargs={'x': 100, 'y': 200}"

See? We got the default value for “b”, because there weren’t any positional arguments to override it.

Keyword-only parameters

We saw earlier that we cannot have a parameter with a default before our *args parameter. Fortunately, Python 3 introduced a new kind of parameter: The keyword-only parameter. Any parameter that comes after *args can only be set with keyword arguments.

There are actually two types of keyword-only parameters: If you define it as name=value, then the parameter has a default value, and doesn’t need to be specified by the caller. But if you just include the parameter name, without a default value, then the keyword argument is mandatory, and calling the function without it will result in an error.

For example:

def myfunc(a, *args, b, c=10):
…: return f'{a=}, {b=}, {c=}, {args}'

If I call it with “myfunc(5,6,7,8,9)”, then we get the following error:

myfunc() missing 1 required keyword-only argument: 'b'

I can then call it with “myfunc(5,6,7,8,9,b=22)” and we get:

'a=5, b=22, c=10, (6, 7, 8, 9)'

Positional-keyword separator

What if you want to have keyword-only parameters, but you don’t want the function to accept *args? Or what if you want your function to only accept keyword arguments (i.e., no positional arguments)? How can you do that?

If you put * in the list of parameters, then all parameters following it are keyword only. In other words, we can define:

def hello(*, first, last):
    return f'Hello, {first} {last}'

We can now call this function as “hello(first=’Reuven’, last=’Lerner’)”. But we cannot call it with any positional arguments.

You can, of course, put the “*” after one or more other parameters. Those preceding parameters can accept either positional or keyword arguments.

Positional-only parameters

The newest kind of parameter is the positional-only parameter. Such parameters, as you can gather from their names, cannot be set with keyword arguments. They are specified by putting a “/” (slash) in the parameter list. Any parameter that comes before a “/” is considered positional only. For example:

def add(a, b, /):
    return a + b

If I call it with “add(10, 3)”, then I get 13. But if I call it with “add(a=10, b=3)”, then I get the following error:

TypeError: add() got some positional-only arguments passed as keyword arguments: 'a, b'

Now, why would you want positional-only parameters? It’s to avoid clashes with keyword arguments. What do I mean by that?

Let’s assume that you want to write a function that takes any keyword arguments and writes them to a file, in the style of a configuration file – with each key-value pair turned into “key:value” in the file. For example:

def write_config(filename, **kwargs):
    with open(filename, 'w') as f:
        for key, value in kwargs.items():
            f.write(f'{key}:{value}\n' )

We can call it as follows:

write_config('myconfig.txt', a=10, b=20, c=30)

But what happens if one of the configuration keys is called “filename”? Let’s try it:

write_config('myconfig.txt', filename='abc', a=10, b=20, c=30)

We get an exception:

TypeError: write_config() got multiple values for argument 'filename'

This is why we have positional-only parameters, so that the keyword arguments don’t clash with the positional arguments. If I redefine my function such that “filename” is positional-only:

def write_config(filename, /, **kwargs):
    with open(filename, 'w') as f:
        for key, value in kwargs.items():
            f.write(f'{key}:{value}\n' )

Now we won’t have any problems calling write_config with a “filename” keyword argument, because Python won’t see it as having been set twice, and kwargs only looks at keyword arguments:

write_config('myconfig.txt', filename='abc', a=10, b=20, c=30)


While Python doesn’t have parameter for different types of values, it does have different sorts of parameters, all of which tell Python whether they will accept positional arguments, keyword arguments, or both. Defining your parameters in the right way will make your code clearer, and make it easier for people who want to call your functions to know how they should do so.

Python allows you to define your function with any or all of the following parameter types, in the following order:

  1. Positional-only parameters, before a /.
  2. Mandatory parameters, which can be passed either positional or keyword.
  3. Optional parameters (thanks to a default value), which can be passed either positional or keyword.
  4. *args, a tuple that contains all otherwise unclaimed positional arguments. You can also put a * here. to indicate that following this point, all parameters are for keyword arguments.
  5. Mandatory keyword-only parameters. The parameter must be named as a keyword argument in order for the function to run.
  6. Optional keyword-only parameters. If the parameter isn’t named in a keyword argument, Python gives it a default value.
  7. **kwargs, a dictionary containing all otherwise unclaimed keyword arguments.

Want to learn more? My free, weekly “Better developers” newsletter brings you this kind of article and information to your inbox every Monday. Check it out at https://BetterDevelopersWeekly.com/.

It’s time to learn Python

I’m on a mission: I want to help people to do more in less time, and to have better careers. How? Using Python.

If you’re reading this, then you might already be convinced that Python is both easy to learn and impressively powerful. And you hopefully benefit from this power, either in your day-to-day work or as you tinker on personal projects.

But a lot of people aren’t quite convinced, or don’t yet know:

  • Non-programmers think that programming is arcane, difficult to learn, or just irrelevant to their work. And besides, the way they currently do things is good enough, right?
  • Experienced developers, meanwhile, wonder why they should learn Python, when they can do just fine with other languages. I often hear this in my corporate training classes — until those hard-core developers see how much can be done with a handful of lines of Python.

I know, from experience, that no matter what your background, Python can help you to become more productive. And the number of companies looking for people with Python experienced continues to grow quickly, which means that having it on your resume gives you a leg up on other candidates.

On Wednesday, I’m holding a free, hour-long Webinar aimed at people who haven’t yet discovered the power and magic of Python. I’ll talk about why it’s a great language, and what people are doing with it. There will also be plenty of time for Q&A.

I hope to inspire people to start learning Python. And yes, I’ll also be pitching my courses as one way to make that move.

You can learn more, and register to join me, at https://FiveReasonsToLearnPython.com/.

Now, if you’re already an old hand at Python, then you probably don’t need convincing. But perhaps you have colleagues who aren’t sure if Python is a worthwhile investment of their time. Or maybe you have friends who have considered it, but aren’t completely sure.

And if you’re new to Python, like more than half of the people getting my free, weekly “Better developers” newsletter? Then you’ll almost certainly learn something new about the language, and ways to use it that you hadn’t considered before.

Questions? Contact me at reuven@lerner.co.il or as @reuvenmlerner on Twitter, and I’ll be happy to answer. But I also hope to see you on Wednesday!

Again, join me, for free, at https://FiveReasonsToLearnPython.com/.

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