More Helpful and Precise Error Messages With Python 3.10

Key Takeaways

welcome to python 310 cool new features my name is chris and i will be your guide in this course you will learn all about the new features packed into the 310 release of python including better error messages structural pattern matching type hint improvements changes to zip and new multi-variable statistics methods python 310 was released on october 4th 2021 and was actively being worked on since may of 2020 the key features of this release are better error messages python aims to be both powerful and beginner friendly up until 3 10 a lot of the syntax error messages have been rather cryptic you might not notice them because you're used to them but if you ever stop to read their contents you might see how challenging it could be for a new coder not only are the messages themselves a little confusing but they often point to the line after the problem was triggered within most error messages there's a little pointer showing where on the line of code the error occurs in addition to improving indications of what line the error happens on 310 also adds more accurate error highlighting with these pointers the big new syntax structure in 310 is the pattern matching statement which introduces two new keywords match and case if you've come from another programming language that supports a switch statement you'll be familiar with the basics but python's implementation goes far deeper than that there's a whole bunch of type hint goodness in python 310. there are improvements to unions aliases and guards as well as better support for type hinting decorators there's also a new introspection call allowing you to get information about annotations used for type hinting at runtime the standard library zip method has a new parameter that makes it safer to use by raising an error if you attempt to zip things of unequal length and there are also three new functions in the statistics library covariance correlation and linear regression and finally there are a whole host of small improvements like default text encodings asynchronous iteration multiple context managers additions to the cis module giving info on the interpreter and dropped support for older ssl in the next lesson i'll show you all the new ways the compiler will tell you you're wrong and that's a good thing in the previous lesson i gave you an overview of the course in this lesson i'll be talking about the error message improvements introduced in python 310. many different kinds of errors raised by the compiler have been improved this includes better indication of where the error happened and better indication of what the error was and where on the line it took place in the top window i'm going to show you python 3 9 while in the bottom window i'll show you the new differences in 310 i'll start out first with a bunch of errors with dictionaries and comprehensions strictly speaking this first one isn't a comprehension but it's very similar it's an embedded generator statements like these must be wrapped in parenthesis to be legal in python 310 the same error gives a better indication of where the problem lies within the statement highlighting the entire generator expression this next one is an actual comprehension to better understand the problem let's first look at some working code this is a dictionary instantiated using the standard library zip function here zip is taking the first item from the first string and combining it with the first item of the second string and creating a tuple it does this for all the items in both strings and then these tuples are turned into key value pairs by the dictionary here is the exact same thing but using the dictionary comprehension instead with me so far okay let's cause a problem like with the embedded generator before parentheses are required this error isn't too helpful it says the syntax is invalid but not y and it points to the four keyword instead of the x y part that actually caused the problem now let's look at this in 310 that's much better isn't it how about some easy to make dictionary mistakes here i've missed the comma after february and the 310 version 310 is much more specific let's do something else this time instead of comma i'll miss the value altogether another very generic message which is far improved in 310. consider this small program in the top window where the month stick is missing its closing brackets let me run this in 3 9 good old invalid syntax and now in 310. you see not only what went wrong but exactly which opening bracket was mismatched now let's look at improvements around exception blocks once again i'm back to python39 in the top window and 310 in the bottom exception blocks can catch multiple exceptions through the use of parenthesis let me show you a working example and now here's what happens if you forget the parenthesis let's try that in 310 in 310 there's a highlight line showing you where the cause is and a nice clear message to go with the syntax error what about if you forget your accept clause that goes with the try block your usual vagueness here and in 310. much improved there are also improvements in other kinds of blocks as well missing colons go from invalid syntax to a specific indication of what you forgot a lot of programming languages allow value assignment inside of a condition block this makes for a tricky to find bug if you accidentally use single equals instead of double you assign the value and that returns true because this problem was so common python decided to disallow assignment in condition blocks python eventually added the walrus operator to allow this but it is still a special operator regular value assignment can't happen in a condition clause here's an example of 3 9 syntax error and now the improvement in 310 it's a nice little hint there to go with it f strings are one of my favorite python features i can't imagine going back to the days of percent s or format although they are very powerful there are still some restrictions within an f string one of those restrictions is the use of the star operator if you haven't seen the star operator before it's used to expand a list in place let me start with a working example consider this list containing the months from quarter one and then another from quarter two you can combine these into the first half of the year list by expanding them with the star operator that's great but you're not allowed to do that in an f string the improvement in 310 isn't a lot but it does explicitly make an indication of the problem not as drastic as some of the other improvements but still useful python's use of white space for coding blocks is the favorite complaint of non-pythonistas however you feel about it the error message when you forget to do it has at least improved here's three nine and a subtle improvement in 310 that indicates the block type that expected the indent some command line terminals now support suggestions when you make a typo now python's reple supports the same concept note that underneath this is all implemented in a method called pi air display and not all repels use this method for error management but for those that do you get the new feature let me import collections and in three nine i'll misspell named tuple i'm going to create a variable named fool and then access foo instead let's try these in 310. did i mean named tuple well yes indeed i did fool me once shame on you fool me twice don't be food again that's it for the errors next up python's answer to switch