CS50P - Lecture 3 - Exceptions

so [Music] [Music] all right this is cs50s introduction to programming with python my name is david malin and this is our week on exceptions exceptions in python as well as in other programming languages refer to problems in your code indeed when something is exceptional in your program it actually doesn't mean it's a good thing it means something has gone wrong that ideally you will somehow solve so what are some of the things that can go wrong so i'm going to go ahead and open up vs code on my computer here and in the terminal window i'm going to go ahead and run code of hello dot pi that's going to of course open up a brand new tab for me hello.pie in which i can write my code and let me go ahead and write some very simple code just to say hello to the world let's go ahead and say print quote hello comma world and then let me go ahead and i'm forgetting to close that quote so a mistake that you yourself might have already made or might surely in the future make and it's a little subtle because you might not necessarily notice that you've just missed that one character well let me go ahead and somewhat optimistically go down to my terminal window now and run python of hello dot pi and hit enter and that's the first of my errors my gosh i've only written one line of code and i seem to have more lines of errors on the screen but the salient point is this bottom most thing here notice where it says syntax error a syntax error is a problem with the code that you have typed your syntax just like english and other human languages have syntax associated with them so does my code and it's not quite correct something is awry i didn't follow the instructions properly and it does elaborate for me unterminated string literal now that's a bit arcane that is a bit of a confusing error message but unterminated would generally mean that i started something but didn't stop it i didn't terminate it string of course is a sequence of text like we discussed before or stir in python and literal generally refers to something that you literally typed it's not a variable it's something like quote unquote or just quote hello world so the fix here of course is going to be to go ahead and terminate that string and actually close the quote and if i now go back down into my terminal window and rerun python of hello.pi now i'm saying hello to the world so the catch with syntax errors here is that syntax errors are entirely on you to solve a syntax error is a problem that you've got to go back into your code and fix from the get go you can't just kind of hope that it's going to resolve itself or expect that other parts of your code will catch it for you syntax errors just must be fixed but there's a lot of other types of errors in python that might be described as runtime errors that happen while your code is running and it's really up to you to write some additional code defensively to detect when those errors happen because you don't necessarily know for instance what input humans are going to type into your program and so you better be ready defensively to accommodate things that they type or even mistype so for instance let's go back over here to vs code and let me propose that we take a look at a new file altogether i'm going to close hello.pi and i'm going to write code of say number dot pi so let's play around with some numbers in python and the first thing i'm going to go ahead here and do with number.pi after opening this new tab is i think i'm going to go ahead and print type up a relatively simple program that maybe prompts the user for an integer like x and then just prints out what x is so we're going to start super simple but again in starting simple we'll be able to really see where i've done something wrong well here we go i'm going to go ahead and say a variable called x is going to get assigned the value of the return value of input quote unquote what's x question mark and i'm going to include a space to move the cursor over a little bit and then ultimately i'm going to go ahead and oh wait a minute if i'm wanting to get an int from the user recall that i need to do something proactively i need to actually convert that input to an integer using the int function in python so now i'm passing the return value of input as the argument to int and that will store an x ultimately an integer not a string that looks like an integer all right let me go ahead now and just quite simply print out what this is i'm going to go ahead and print out quote unquote x is x but i don't want to literally say x is x i want to plug in the value of x so maybe the easiest way to do that is to surround it with curly braces and then if i'm using these curly braces and i want python to interpolate the value of that variable that is substitute what x actually is in between those curly braces recall that i need to use a format string or an f string by prefixing this whole thing with an f now that i've done that let's go ahead and see what happens i'm going to go ahead in my terminal window and run python of number.pi i hit enter and so far so good all is well i'm being prompted for x let me go ahead and type in a number like 50. all right that seems to work program seems to be correct or is it what could go wrong in this program even though nothing did just go wrong but if i run it and run it and run it again during the running of my program what could still go wrong especially if i'm not the human interacting with it but some other human instead any volunteers here for this one what could go wrong and in what way is this program not really correct even though at first glance it seems so what uh what is any interpreter might be code in a in a integrated ide we can't code in a interpreter so i'm not calling an integer i'm still having trouble hearing you but what i think i heard is that if what the user types in is not in fact an integer i can't just blindly convert it to an int if i'm not putting too many words into your mouth i think what i should perhaps do here is be a little defensive and let me see if i can't simulate exactly the problem that could go wrong here let me go ahead and run again python of number.pi let me try another number and in fact when testing your code generally it's a good idea to test corner cases maybe numbers that aren't quite as plain as 50 or 49 or 51 let's choose some numbers that might be a little more interesting if only mathematically like zero all right zero seems to work my code still prints out that x is 0. what might be another corner case to consider well let me go ahead and try a negative number that 2 is pretty different in spirit from 50. negative 1 okay that works too well let me try it one more time i've tried positive numbers negative number zero let me try something like a cat so literally c-a-t typing in a string that doesn't even look like a number and yet let's see now what happens when i hit enter all right we'll see now we've got another kind of error it's not a syntax error because i didn't make a typographical mistake i didn't forget some piece of syntax i actually now have a error with one of my values and it's in a value i didn't even anticipate the human me in this case typed it in long after i wrote the code so what does this refer to a value error well let's see what the explanation is invalid literal for int with base 10 quote-unquote cat now this too is a bit of a mouthful and unfortunately in python in a lot of programming languages the error messages are written for pretty comfortable programmers and of course when you're learning programming for the first time you might not be so comfortable with the programming language let alone the error messages but let's see if we can't glean some insight so invalid literal well again a literal is just something that's been typed in it would seem for int what is int exactly well int is the function i'm using to convert the user's input to a corresponding integer base 10 that refers to the decimal system which is the default that python's using and it looks like at the end of the day what python really doesn't like is that i passed cat quote unquote to the int function so how do i go about actually fixing this problem well i could just add instructions in my program maybe i could add a line of print telling the user more explicitly be sure to type an integer or please don't type cat please don't type strings of course the user might still not oblige they might not be reading the instructions so that too is probably not an effective strategy what we really want to do is write our code with error handling in mind we want to write lines of code that not only accomplish the problems we care about but that also handle errors that might unexpectedly happen and in general when programming programming defensively assume that the users aren't going to be paying attention or worse they're malicious they're trying to crash your program so we want to handle as many errors as we can now how do we go about doing that in python well it turns out whether you want to catch a value error or other types of errors as well though not syntax error python actually has this keyword called try and it's sort of aptly named if you want to try to do something in python you can literally use this keyword and you can check whether or not something exceptional something erroneous has happened so using both try and this other keyword accept can i go and try to do something except if something goes wrong i can do something else instead so let's consider how can i go about trying to convert the user's input to an end except if something goes wrong well let me go back to my code here and let me propose that i now modify this example as follows let me go ahead and above my first line of code i literally write try and a colon telling python try to do the following i'm going to go ahead and indent my existing lines of codes here by the same number of spaces four in this case and then i'm going to add one more new line down here that literally says accept value error and notice it's important that i've capitalized the v and i've capitalized the e these symbols are case sensitive and this is now an opportunity after this colon to tell python what i want to do in exceptional cases when the number or the input from the user is not in fact a number and i'm going to say something plain like print quote unquote x is not an integer i'm at least going to tell the user roughly what the problem actually is so notice another detail the indentation is important because i have try on line one and i've indented lines two and three those are the two lines of code that i'm trying except if i see a value error line 5 because it's indented is what is going to get executed in cases of those errors let me go ahead now back to my terminal window and run python of hello of python of number dot pi enter and let's go ahead and type in 50 again still seems to work and of course i'm trying and succeeding let me go ahead and try once more this time though with the word cat or really anything that's not a decimal number and now you'll see much more cleanly x is not an integer i'm not seeing some scary error message that i of a user am i'm going to have no idea how to handle now you the programmer have anticipated that something exceptional can happen and you've gone about actually handling the error for the user giving them an appropriate error message instead let me pause here and see are there any questions now on what we've just done by introducing try and accept to handle this value error is value ever the only type of ever you can get or the other types is value-added the only thing you can catch there are other errors as well and we'll see a few of them today and there's many many more honestly that if you continue programming and programming in python you're going to see a lot of them over the weeks the months the years to come but the technique for handling them is going to be largely the same other questions on try accept or these exceptions more generally yes sir actually to use the accept block you need to know the type of error right like here you knew it for the value error what is what if you can't anticipate this particular type of error a really good question so i'm being very good about catching so to speak the very error that i know might happen i don't know when it might happen because it's going to depend on the user but i know what kind of error will happen from the int function there is a way in python where you can say except if anything goes wrong and you can literally omit value error and just catch everything the problem with that is that it sometimes hides other bugs in your code because you don't necessarily know what's going wrong and if you don't necessarily know what's going wrong how can you possibly handle it correctly so bad practice and put another way it's lazy to do that to just say catch everything and i'll deal with it here so a much better practice would be to figure out what kind of errors could happen and include mention of them explicitly as i've done now with that said if you read python's official documentation as you'll eventually invariably do it is not great about telling you proactively what kinds of errors can be raised in this way so it's a bit of contradictory advice you should do it this way but it's not always obvious what you should be checking for but you get better at it with practice and some of the times the documentation does spell out what could go wrong let me turn our attention now back to this and point out that even though this is better code it is more correct in the sense that i'm not just leaving it to the user to see some really ugly default python error message that most people are going to have no idea what to do with i'm at least handling it more elegantly and i'm printing out x is not an integer so it's at least more instructive but this isn't necessarily the best way to implement this code why well here too i'm actually still being a little lazy so notice that i'm trying to do not one line of code but two lines of code and this isn't a huge deal because we're only talking about two lines of code but in the interest of pr preaching best practices you should really only be trying to do the one or very few lines of code that can actually raise an exception that can actually fail in some way i am pretty sure that calling print here is not going to raise a value error whether x is an int or a string or a float or anything else the format string feature of python is going to handle printing it just fine so really what i'm going to do is this i'm going to move this line 3 down to the bottom of my code i no longer need to indent it i'm just going to execute it at the bottom of my file here unfortunately by doing this i've done a good thing by now only trying to do the minimal amount of work necessary that might raise the exception a value error but i fear i've introduced a new mistake well let's see what is now incorrect let me go ahead and again run python of number.pi enter let me go ahead and do it correctly with 50 and all seems to be well but again let's try those corner cases the zeros the negative numbers or in this case the cat let me go ahead and type in cat again enter now i have a name error so now it's yet another type of error in my code that i've introduced here and what is this name error mean well just as a value error refers to that the value of some variable the value that someone has typed in is incorrect name error tends to refer to your code like you're doing something with the name of a variable that you shouldn't and why might that be well let me turn our attention back to the code here and consider what is it complaining about well the name error is what i see down here and it's telling me name quote unquote x is not defined and notice if i look further here it is mentioning line six so i know the problem is with my code on line six and that worked a moment ago and i'm defining x on line two but let me ask the group here why does x not in fact exist on line six why is it not defined even though i'm pretty sure i was intending to define it on line two maybe the scope of the variable is between the try block so good terminology scope refers to the portion of code in which a variable exists that too though isn't quite right in python that would be true in cc plus plus and java where indentation or curly braces tend to define the scope of a variable but again here in general and this worked a moment ago x exists once it's defined on line two because remember i printed out x is 50 a little bit ago let's try one more hypothesis here one more hand why is x somehow still not defined um yeah so is it because it's local variable meaning that like it doesn't define outside of scope it's like what people have mentioned it's it's asked it prompts the input and try right but outside of it it's undefined so still good instincts and good terminology too there's this notion of local variables which tend to exist inside of functions for instance global variables which tend to exist in entire files in this case too though that's not quite the case what's happening here boils down to order of operations let me come back to the code here and recall that anytime we've discussed the assignment operator the single equal sign that copies a value from the right to the left but consider for a moment at what point something is going wrong well the input function is probably working just fine because we've used that a lot now to get user's input it always returns a string or a stir in python but what could be going wrong well if i'm passing that string to the int function as its argument it's probably the int function that's airing and indeed if you think back earlier when we had the value error it was in fact the in function that did not like quote unquote cat as input so this is all to say that this portion of my code highlighted now to the right of the equal sign that's the code that's creating a problem that's the code that was creating a value error and in this case we're catching the value error but because the value error is happening on the right of the equal sign there's no value being copied to the left the error is interrupting that whole process so even though we see x equals dot dot dot on line two the portion of that line to the left of the equal sign isn't getting evaluated ultimately because the value error is happening too soon and so when we finally get down to line six even though it looked like i was defining on line two and i would have defined x on line two if all had gone well we didn't get to the part where the value is copied from right to left because the value error happened first so this code is just incorrect now so how do i go about solving something like this well it turns out that there's another feature of the try and accept syntax that python supports which is that it also supports the keyword else now we've seen else before if you think back to our discussion of conditionals we saw if we saw l if we saw else which was kind of this catch-all what you should do in the event that nothing else is relevant that's kind of the same intuition here for the try accept feature of python what you can do is this you can try to do the following as i've done except if this goes wrong but if nothing goes wrong else go ahead and do this so this is one way i can solve this same problem now no matter what now python is going to try to execute line two if something goes wrong it's going to execute slides 3 and 4 to handle that value error however if you try and this code succeeds then there is no exception to handle so you're then going to execute this line here so it's a little confusing perhaps in that we're now using else both for conditionals if l if l if l if else and we're also using else with these try accept blocks but that's okay that's part of the language that's one of the features so now if i rerun this code in my terminal window python of number.pi let's do something correct like 50 i see that x is 50. so line 1 is executed we're trying to do the following line 2 is executed because the conversion happens successfully and the number 50 gets copied from right to left the exception does not happen so we ignore lines three and four we jump immediately to line five and six which prints out the result by contrast though let's do this one last time python of number.pi let's type in cat or again any other word and hit enter now we don't see what x is rather we see quote unquote x is not an integer which is what's being handled in my accept clause all right let me pause here because that's a lot of new syntax and see here if there's any questions on try on accept on else name error or value error can you please repeat try function repeat the name error what's the problem with the name error yes yes yeah so let's let's just rewind a couple of lines here before i fix this problem by now getting rid of the else a moment ago we had code that looked like this whereby i was getting a name error python of number.pi enter typing in cat that looked like this where namex is not defined and the problem was on line six according to this output in python well let's think about this now deductively let's try a different approach on line six i'm seeing an error that name x is not defined okay python's already telling me x does not exist at that point so how could that possibly be well where should x be defined well presumably x is defined on line two up here so what could go wrong well if the user has inputted something that doesn't look like a number like the word cat passing cat the return value of input as the argument to int to convert the word to an in makes no sense you can't convert a cat cat to an integer at all so the int function is raising a value error at that point and the error is being handled with this code here but notice this line 6 is not indented it's left aligned with the rest of my code which means no matter what line 6 is going to execute it's going to execute whether i typed in 50 or i typed in cat but if i typed in cat again x never gets a value so it's not defined here on line six so when i introduce finally the else statement that makes sure that these things are mutually exclusive i only execute the else if i tried and succeeded up above well let me propose that we refine this just a little bit further as well and consider how we might improve this example a little bit more it's a little it's a little unfriendly of me to be rejecting the user's input after they fail to provide an integer and just quitting the program really right it'd be more user-friendly if i just prompt or re-prompt the user again and again and in the chat if you could what's the feature of python that you can use if you want to do something again and again and again until such time as the user cooperates and gives you what you're looking for like a number so yeah loop loop loop so a loop is something that happens again and again and again and maybe we can use that same mechanism a loop in order to prompt the user for x and if they don't give us a number prompt them again and if they don't prompt them again and again and again we don't need to just quit out of the program so quickly so let me propose this let me propose here that i improve this code by deliberately doing this let me induce a infinite loop at the very top of my code with while true recall that the while keyword induces a loop a cycle that behaves like this and it asks a question a boolean expression that needs to evaluate either to true or false well if i want this thing to loop forever at least initially we'll just say while true because true is true so this has the effect of doing something no matter what forever unless we break out of it early now i'm going to go ahead and do this i'm going to go ahead and move my try accept code indented underneath this loop so that i'm trying to get an x if i have a value error instead i print that x is not an integer but this time what do i want to do if the user does try and succeed in giving me a number while i can do this i can just break out of my code here and down here now i can use that same line of code from before an f string that says x is and then in curly braces x again so what's going on here i think this code now because i've added the loop is going to have the effect of trying at least once maybe a second time maybe a third time maybe 500 times until the user finally gives me what i want which is an integer and once they do once there's no value error happening then i break out of the loop and line 9 executes as i would hope so let me go ahead and try executing this version python of number dot pi enter what's x let me go ahead and type in the easy thing first 50. x is 50. what just happened in terms of the control flow of this program the flow of my logic well i first found myself on line one inside of a loop hopefully i'll get out of this loop what did i then do on lines two and three i tried to get input from the user and convert it to an end well i was a nice guy this time and i typed in 50 which looks like and is a number so the in function converted it just fine and stored it from right to left in x accept value error there is no value error because if i typed in a number there's nothing exceptional happening this is a boring good execution of my program so what happens i break out of the loop so again the else clause is associated with the try not with the except and once i'm out of the loop of course i'm just printing out what x is well let's try the other scenario that might happen python of number.pi enter what's x let's try cat or any other word enter ah this is now a new feature i'm being informed what i did wrong x is not an integer so i'm getting some useful user feedback but notice again i'm prompted what's x well let me try typing in dog x is not an integer what's x let me try bird enter x is not an integer what's x and suffice it to say this will happen now forever if i'm in an infinite loop until i try and succeed at which point i break out so let's try again 50 enter now i'm out of the loop and i'm printing out what x actually is all right let me pause here and see if there are any questions the logic is almost the same but what is different now is i'm in a loop and i'm using the keyword break in python to deliberately break out of the loop when i'm ready to once the user has cooperated we really need to break can't we just print or what keeps us from just printing uh good question so let me try that couldn't i just print well let's see what happens if i do that let me move this print line at the end into my loop here thereby shortening the program and in general that's been a good thing python of number.pi enter let me go ahead and type in 50. okay x is 50. what's x uh okay maybe it's 49. x is 49. uh okay maybe 48. unfortunately i think you're you're laughing you see it i never break out of the loop which maybe that's a feature maybe you want this to be your program but i didn't i'd eventually like this game to stop so i need to break out in that way but i can do it a little differently and let me propose that we modify this a little bit but first any other questions on this syntax here let me rewind to the prior version hey can i use break uh only exception except in else for example in another uh print and when you use printing the else you can use print together with break or something like this you so you can use break inside of loops to break out of loops and you can use it inside of a conditional like an if an lift or an else you can do it inside of a try accept else statement too anytime you're in a loop that you want to break out of you can use this keyword break i'm using it in the context of exceptions but it's not restricted to that and let me show you too it doesn't even have to be in the else if i wanted to i could actually do this i could get rid of my else and i could go back to line three add another line that's indented line four and break out here now why is this logically okay well consider what i'm now trying to do i'm trying to execute line three and converting the user's input to an int and i'm trying to store the result from right to left in x if something goes wrong the code we've already seen is immediately going to jump to line 5 and then 6 to handle the exception but if nothing goes wrong my code presumably should just keep on executing line by line so i could technically logically put the break here and watch what happens when i run this version python of number dot pi 50 enter it worked i broke out of the loop now which way is better honestly i think it could go either way at this point this program's so relatively short that even though i'm trying to do two things now one of which the break is not going to fail like you either break or you don't there's no piece of data from the user that's going to influence that we don't strictly need to have those two lines of code there but it's only two lines so i think it's okay and if you recall our discussion in the past not just of correctness does the code work as it should but design i think you could argue it either way if you prefer the readability of this and the fact that you don't have an else that's fine if though you prefer to minimize just how many lines of code you're trying to execute in case something goes wrong the else is a reasonable approach too well allow me to propose two now that we refine this further i think we're at the point where it's pretty darn correct but suppose now that i find myself today and tomorrow trying to get numbers from the user quite a bit it would be nice as we've seen to maybe just invent my own function getint to get an integer from the user both today and tomorrow and beyond and heck maybe i can even share that function with other people if they want to write programs they get integers from users so how might i go about doing this well let me go ahead and propose that we do this let me get rid of the print line but keep most of my loop here let me define a function called getint that takes no arguments for now and i'm going to go ahead and indent all of the code i already wrote underneath getint so now i have a function called getint that tries to do the following try to get in it from the user if something goes wrong and there's a value error yell at them with x is not an integer else break but it's not just breaking that i want to do here now that i'm in a function recall our discussion of return values if you're inventing your own function whose purpose in life isn't just to print something on the screen like a side effect but is to hand back a value to hand you back a value like on that same post-it note from our discussion of functions well you need to return x explicitly how do i now use this function well as soon as we start making our own functions it tends to be convenient to define our own main function as well that's the main part of our program and i'm going to keep this simple i'm now going to say x equals get int and then on the next line i'm going to do that print from before quote unquote x is and curly braces x and at the very bottom of my program recall i'm going to call main so that no matter what i'm invoking my main function after everything's been defined well let's see how this works let me go ahead and run python of number.pi enter let's type in 50 and it seems to work as before let's go ahead and run it again typing in cat cat this time x is not an integer and i'm being prompted dog and i'm being prompted bird and i'm being prompted fine fine fine 50. that's an int and so it is printed so what's worth noting here while i'm manifesting a couple of good properties here one i've kind of abstracted away this notion of getting an integer and even though i just artificially hit enter a whole bunch of times just to hide that function for now it needs to be there but we don't need to see it at this point notice that now this entire program really boils down to just these three lines of code now why because i've abstracted away that whole process of getting an int from the user into this new function of my own called get int but can i improve upon this well let me go ahead and undo all of those blank lines and pull this up just so we can see more on the screen at once can i tighten up my implementation of getint it is correct i claim this is correct it's handling errors and it's returning x but i don't strictly speaking need to write the code as long what else could i do well let me propose that if all you're doing on this line 13 is breaking and then immediately after that per the indentation you're executing return x on line 14. why are you wasting everyone's time once you know you're ready to return the value you could just return x and so in my else i could break out and return a value so here too return is used to return values from functions break is used to break out of loops but it turns out that return is sort of stronger than break it will not only break you out of a loop it will also return a value for you so it's doing two things for once if you will but can i make this even more compact if if if my goal is to just tighten the code up even though it's already correct can anyone think of a further refinement whether you've programmed in python before or not can i shorten this implementation further just a little bit if only to decrease the probability that i've made a mistake by having fewer lines and just make it a little easier to read because it's shorter any suggestions for tightening up my implementation of getint you can just return the value on the try function when you're trying uh you take the input x and then return x good we can just return x a little higher up and let me correct folks as we go it's not a try function it would be a try statement technically a function typically has a parenthesis and another one in this case it's just a statement so but we can do exactly that i don't technically need the else if i really want i could do this right after line 9 i could return x here or recall our discussion of defining variables unnecessarily sometimes like why define a variable here if you're immediately going to use it here and then never again so we could avoid a new line here and i could avoid even defining x explicitly i could just say something like this i could return int input quote unquote what's x i can do it all at once now which is better i don't know i mean again this is where reasonable people might disagree i'd argue that on the one hand we're tightening up the code we're using fewer lines it's easier to read lower probability that i've made a mistake on the other hand it's a little more complicated to understand perhaps it's a little less obvious where i'm returning from so i think arguments can be made either way at the end of the day what's important is that you've done this consciously you've made a decision to do it this way or this way and you can justify it in your mind not that your answer is it worked so i left it alone like have a good reason come up with a good reason and that will come with experience and practice well let me propose to that we make one other refinement here suppose that you're finding your programs to be a little noisy and it's a little obnoxious that you keep telling the user x is not an integer x is not an integer x is not an integer what if you want to make things a little gentler and just prompt the user again with the same words what's x what is x what's x again and again well you can do that as well and it turns out that if you want to handle an exception in python but you want to pass on doing anything with it so you want to catch it but you essentially want to ignore it you don't want to print anything you don't want to quit the program you just want to silently ignore it like if you're talking in a groom full of people and you're it's your turn to talk and you're just like pass they're still calling on you but you're not doing or saying anything more well we can add this keyword to our code here let me go back to my program here and instead of printing out again and again x is not an integer i could just do this i could pass on handling the error further i'm still catching it so the user is not going to see a scary message even mentioning value error my code is catching it but i'm passing on saying anything about it i'm going to stay in the loop i'm going to stay in the loop and keep prompting and reprompting the user so now the effect looks a little something like this python of number.pi let's type in cat what's x again let's type in dog what's x again type in bird so it's just a little maybe more user friendly and that you're just reminding the user what you want maybe it's worse maybe it would be helpful to tell the user why you're prompting them again and again it's not obvious so it could go both ways but again it's just another mechanism now for handling these errors we use the accept keyword to catch a specific error but we don't have to handle it more than that we can just pass on doing something further let me pause here and see if there's any questions now on try accept else or pass okay yeah no i was just kind of curious i guess about the idea of when you were indenting with uh the get in function for example um because i'm noticing you know obviously going through it with the whole logic and breakdown of the entire uh the entire function you know while true do this but i'm just kind of curious on elaborating with the indentations for the code more yeah so the indentation is deliberate logically some languages don't require as rigorous indentation you can use curly braces or other symbology to make clear what is associated with what in general anytime you indent something in python on this line so rather anytime you write a code a line of code in python that's here and the lines below it are somehow indented that means that those lines are somehow associated with that first line and presumably those indented lines should only be executed if the first line told the uh told the computer to do so so concretely what does this mean on line six here we're defining a function called get in that takes no arguments colon everything that's indented by at least four spaces hereafter is part of that function why that's just the design of the python language frankly i think the designers got tired of seeing really ugly code in languages like c and c plus and java that don't necessarily enforce uh indentation to this extent so now it's baked into the language and my chronology might be a little off there but there's been many languages that are looser than python when it comes to indentation the indentation is meaningful on line seven two notice that because the while true is indented by four spaces that just means it's part of the get in function but notice below the while true statement there's 8 there's 12 there's 8 there's 12 spaces here and i'm just quickly counting the dots that means that all of the lines i've just highlighted are inside of that while loop while true means to execute lines 8 through 11 potentially again and again and again and now lastly on line eight because we have try and indented below it is line nine that just means that what you should try is what's on line nine and similarly on line ten below it we have indented line 11. you should only pass when there is an exception of a value error so the indentation just means what is associated with what and once you get comfortable with that you'll see that it helps the indentation alone helps explain the logic of your program and it has a wonderful side effect that for yourself the next morning for your colleagues your family your friends your teachers your code is much more readable as a result it's not one big mess of a blob of text other questions now on try accept else or pass yeah thanks um two question uh question one um once you say pass can the caller still learn anything about this error um through a system variable or whatever and question two problem set zero referenced some string methods including is numeric is it any different to um go via is the merrick here good question so on the first question if i'm handling the error in this way the caller is not going to know anything about it that's the point of my handling it so that maine or other callers don't know that anything technically went wrong on the second question is numeric is another function that you can call that can look at a string and determine is this in fact a number i could use a mechanism like that i could use a conditional if this looks like a number then pass it to the int function and go ahead and convert it to an integer that's totally fine i would generally say that the pythonic way of doing things is often for better for worse to try things hope they work but if they don't handle the exception so other languages are more in favor of checking if if if if elif else in all of these conditionals python tends to be a little more of the mindset try it but just make sure you're handling the error so this would be the pythonic way of doing it your way though checking with a conditional is it a number first is totally reasonable too if you want to go that way well let me propose some final refinements to this program that really just kind of tighten things up one additional step to improve the implementation of this get in function let me propose that we not hard code so to speak that is type manually x all over the place let's make this function get int a little more reusable right now notice that i'm just kind of using the honor system that well maine is defining a variable called x and getint is asking for a variable called x but it would be nice if the caller main doesn't have to know what the callee is naming its variables and vice versa so caller to call a function means to use it the caller is the function that's using it the callee is just the function being called it would be nice if i'm not just hoping that x is the same in both places so let me propose this let me propose that we actually add a parameter to getint like this what's x that is to say if main wants to use the get in function well then main should probably tell the get in function what prompt to show the user just like the input function recall that comes with python it's up to you to pass in a prompt that the user then sees when the human is asked for input so how do i make this work here i can go down to my definition of get int and i can say all right getint is going to take a parameter now called prompt i could call it anything i want but prompt in english is pretty self-explanatory it means what do you want the message the user will see and now down here when i actually use input i don't have to presumptuously say what's x because what if the program the caller wants to ask for y or z or some other variable i can just pass to input whatever prompt the caller has provided so now i'm making more reusable code it still works just the same i haven't changed the functionality per se but now it's a little more dynamic because now getint doesn't have to know or care what variable is being asked for what's being asked for it just needs to know what prompt it should show to the user so if i now run this program down here again prompt number.pi enter what's x 50 still seems to work let's run it again let's type in cat it still seems to work and if i type in cat dog bird or anything else it will keep prompting me with that same prompt making this code therefore all the more usable now it turns out too you can even raise exceptions yourself using python's raise keyword but more on that another time so in the coming days the coming weeks the coming months as you write more code in python you'll see that errors are inevitable sometimes there's syntax errors which you gotta just fix if you even want to run your program at all but they could be name errors for instance variables that you meant to define but somehow didn't value errors where maybe the user didn't cooperate and provided you with something that you weren't expecting or a whole list of other possible errors or exceptions but now hopefully you know how you can handle these errors and respond to them in any way you like this then was our look at exceptions and we'll see you next time

Channel: CS50

Views: 63,115

Keywords: cs50, harvard, computer, science, david, malan

Id: LW7g1169v7w

Length: 44min 36sec (2676 seconds)

Published: Fri Jul 15 2022