Wiley Beginning Python
Contents
1. ll naturally start to think more about how this works In this case the print function is an example of a built in function which is a function included as a part of Python as opposed to a function that you or another programmer has written The print func tion performs output that is it presents something to the user using a mechanism that they can see such as a terminal a window a printer or perhaps another device such as a scrolling LED display Related routines perform input such as getting information from the user from a file from the network and so on Python considers these input output I O routines I O commonly refers to anything in a program that prints saves goes to or from a disk or connects to a network You will learn more about I O in Chapter 8 Summary In this chapter you ve begun to learn how to use the programming editor codeEditor which is a pro gram written in Python for the purpose of editing Python programs In addition to editing files 10 Programming Basics and Strings codeEditor can run a Python shell where you can experiment with simple Python programming lan guage statements Within the shell you have learned the basics of how to handle strings including adding strings together to create longer strings as well as using format specifiers to insert one or more strings into another string that has format specifiers The format specifier s is used for strings and it can be combined wi2. the same style of quote in which you enclosed the entire string you need to escape the quote that encloses the string to prevent Python from thinking that it has prematurely reached the end of a string If that sounds confusing it looks like this gt gt gt And he said this string has escaped quotes And he said this string has escaped quotes Returning to those three examples normally a running Python shell will show you a string that it has evaluated in single quotes However if you use a single quote within a string that begins and ends with double quotes Python will display that string with double quotes around it to make it obvious to you where the string starts and where it ends gt gt gt Ben said How re we supposed to know that Ben said How re we supposed to know that SoS This shows you that there is no difference between single and double quoted strings The only thing to be aware of is that when you start a string with a double quote it can t be ended by a single quote and vice versa Therefore if you have a string that contains single quotes you can make your life easier by enclosing the string in double quotes and vice versa if you ve got strings with quotes that have been enclosed in single quotes SQL the language that is used to obtain data from databases will often have single quoted strings inside of them that have nothing to do with Python You can learn more about this when you reach Chapt
3. you want to print at once A simple example is when you have sepa rate records of a person s first name and last name or their address and you want to put them together In Python each one of these items can be treated separately as shown here gt gt gt John John gt gt gt Od OQ gt gt gt Public Public gt gt gt Try It Out Using to Combine Strings To put each of these distinct strings together you have a couple of options One you can use Python s own idea of how strings act when they re added together SS Waele a We oak PU DIEE JohnQ Public How It Works This does put your strings together but notice how this doesn t insert spaces the way you would expect to read a person s name it s not readable because using the plus sign doesn t take into account any con cepts of how you want your string to be presented You can easily insert spaces between them however Like newlines spaces are characters that are treated just like any other character such as A s d or 5 Spaces are not removed from strings even though they can t be seen gt gt gt John om omg nO om omg Public John Q Public After you determine how flexible you need to be you have a lot of control and can make decisions about the format of your strings Programming Basics and Strings Putting Strings Together in Different Ways Another way to specify strings is to use a format specifie
4. Different Quotes Enter the following strings keeping in mind the type of quotes single or double and the ends of lines use the Enter key when you see that the end of a line has been reached gt gt gt This is another string This is another string gt gt gt This is also a string This is also a string gt gt gt This is a third string that is some how different This is a third string that is some n how different How It Works If you use different quotes they may look different to you to the Python interpreter however all of them can be used in the same situations and are very similar For more information read on These examples raise a few questions In your first text example you saw that the text was enclosed in double quotes and when python saw two quotes it repeated those double quotes on the next line However in the preceding example double quotes are used for This is another string but below it single quotes are used Then in the third example three double quotes in a row are used and after the word some we used the Enter key which caused a new line to appear The following section explains these seemingly arbitrary conventions Understanding Different Quotes Three different types of quotes are used in Python First there are the single and double quotes which you can look at in two ways In one way they are identical They work the same way and they do the same things Why ha
5. Programming Basics and Strings This chapter is a gentle introduction to the practice of programming in Python Python is a very rich language with many features so it is important to learn to walk before you learn to run Chapters 1 through 3 provide a basic introduction to common programming ideas explained in easily digestible paragraphs with simple examples If you are already an experienced programmer interested in Python you may want to read this chapter quickly and take note of the examples but until Chapter 3 you will be reading material with which you ve probably already gained some familiarity in another language If you are a novice programmer by the end of this chapter you will have learned some guiding principles for programming as well as directions for your first interactions with a programming language Python The exercises at the end of the chapter provide hands on experience with the basic information that you ll have learned How Programming Is Different from Using a Computer The first thing you need to understand about computers when you re programming is that you control the computer Sometimes the computer doesn t do what you expect but even when it doesn t do what you want the first time it should do the same thing the second and third time until you take charge and change the program The trend in personal computers has been away from reliability and toward software being built on top of other unrel
6. built from fundamentally understandable and controllable pieces Programming Is Consistency In spite of the complexity involved in covering all of the disciplines into which computers have crept the basic computer is still relatively simple in principle The internal mechanisms that define how a com puter works haven t changed a lot since the 1950s when transistors were first used in computers In all that time this core simplicity has meant that computers can and should be held to a high stan dard of consistency What this means to you as the programmer is that anytime you tell a computer to metaphorically jump you must tell it how high and where to land and it will perform that jump over and over again for as long as you specify The program should not arbitrarily stop working or change how it works without you facilitating the change Programming Is Control Programming a computer is very different from creating a program as the word applies to people in real life In real life we ask people to do things and sometimes we have to struggle mightily to ensure that our wishes are carried out for example if we plan a party for 30 people and assign two of them to bring the chips and dip and two of them to bring the drinks With computers that problem doesn t exist The computer does exactly what you tell it to do As you can imagine this means that you must pay some attention to detail to ensure that the computer does just w
7. d the instructions that you ve put in there become the actions that the computer takes Just as authors and editors have specialized tools for writing for magazines books or online publica tions programmers also need specialized tools As a starting Python programmer the right tool for the job is codeEditor Starting codeEditor Depending on your operating system you will start codeEditor in different ways Once it is installed on your system with PythonCard on Linux or Unix based systems you can just type codekditor in a terminal or shell window and it will start On Windows codeEditor should be in your Start menu under Programs PythonCard Simply launch ing the program will get you started When you start codeEditor for the first time it doesn t display an open file to work with so it gives you the simplest possible starting point a window with very little in it Along the left side you ll see line numbers Programmers are often given information by their programs about where there was a problem Chapter 1 or where something happened based on the line number in the file This is one of the features of a good programming editor and it makes it much easier to work with programs Using codeEditor s Python Shell Before starting to write programs you re going to learn how to experiment with the Python shell For now you can think of the Python shell as a way to peer within running Python code It places you insid
8. e of a running instance of Python into which you can feed programming code at the same time Python will do what you have asked it to do and will show you a little bit about how it responds to its environ ment Because running programs often have a context things that you as the programmer have tai lored to your needs it is an advantage to have the shell because it lets you experiment with the context you have created Sometimes the context that you re operating in is called your environment Try It Out Starting the Python Shell To start the Python shell from codeEditor pull down the Shell menu in the codeEditor s menu bar and select Shell window This will open a window with the Python shell in it no surprises here that just has simple text with line numbers along the left side see Figure 1 1 You can get a similar interface without using PythonCard by starting the regular Python interpreter without PythonCard s additions by just typing python on a Unix system or by invoking Python from the Start menu on a Windows system Untitled Code Editor PythonCard Applicatio File Edit View Format Shell Help Ab Figure 1 1 Programming Basics and Strings After you ve started the shell you ll be presented with some information that you don t have to be con cerned about now from import pcapp and so on followed by the sign that the interpreter is ready and waiting for you to work with it gt gt gt gt gt g
9. er 14 One more important rule to know is that by themselves quotes will not let you create a newline in a string The newline is the character that Python uses internally to mark the end of a line It s how computers know that it s time to start a new line Within strings Python has a way of representing special characters that you normally don t see in fact that may indicate an action such as a newline by using sequences of characters starting with a backslash Remember that it s already special because it s the escape character and now it s even more special The newline is n and it is likely the most common special character you will encounter Until you see how to print your strings you ll still see the escaped characters look ing as you entered them as n instead of say an actual line ending with any more tests starting on the next line Python has one more special way of constructing strings one that will almost always avoid the entire issue of requiring an escape character and will let you put in new lines as well the triple quote If you ever use a string enclosed in three quotes in a row either single or double quotes but all three have to be the same kind then you do not have to worry about escaping any single instance of a single or dou ble quote Until Python sees three of the same quotes in a row it won t consider the string ended and it can save you the need to use escape charac
10. hat you want it to do One of the goals of Python is to program in blocks that enable you to think about larger and larger pro jects by building each project as pieces that behave in well understood ways This is a key goal of a pro gramming style known as object oriented programming The guiding principle of this style is that you can create reliable pieces that still work when you piece them together that are understandable and that are useful This gives you the programmer control over how the parts of your programs run while enabling you to extend your program as the problems you re solving evolve Programming Copes with Change Programs are run on computers that handle real world problems and in the real world plans and cir cumstances frequently change Because of these shifting circumstances programmers rarely get the opportunity to create perfectly crafted useful and flexible programs Usually you can achieve only two of these goals The changes that you will have to deal with should give you some perspective and lead you to program cautiously With sufficient caution you can create programs that know when they re Programming Basics and Strings being asked to exceed their capabilities and they can fail gracefully by notifying their users that they ve stopped In the best cases you can create programs that explain what failed and why Python offers especially useful features that enable you to describe what conditions may ha
11. iable software The results that you live with might have you believing that computers are malicious and arbitrary beasts existing to taunt you with unbearable amounts of extra work and various harassments while you re already trying to accomplish something If you do feel this way you already know that you re not alone However after you ve learned how to program you gain an understanding of how this situation has come to pass and perhaps you ll find that you can do better than some of the programmers whose software you ve used Chapter 1 Note that programming in a language like Python an interpreted language means that you are not going to need to know a whole lot about computer hardware memory or long sequences of Os and 1s You are going to write in text form like you are used to reading and writing but in a different and sim pler language Python is the language and like English or any other language s you speak it makes sense to other people who already speak the language Learning a programming language can be even easier however because programming languages aren t intended for discussions debates phone calls plays movies or any kind of casual interaction They re intended for giving instructions and ensuring that those instructions are followed Computers have been fashioned into incredibly flexible tools that have found a use in almost every business and task that people have found themselves doing but they are still
12. r It works by putting in a special sequence of characters that Python will interpret as a placeholder for a value that will be provided by you This may initially seem like it s too complex to be useful but format specifiers also enable you to control what the displayed information looks like as well as a number of other useful tricks Try It Out Using a Format Specifier to Populate a String In the simplest case you can do the same thing with your friend John Q Sos Mon Oo Se amp Public John QO Publica How It Works That s is the format specifier for a string Several other specifiers will be described as their respective types are introduced Each specifier acts as a placeholder for that type in the string and after the string the sign outside of the string indicates that after it all of the values to be inserted into the format speci fier will be presented there to be used in the string You may be wondering why the parentheses are there The parentheses indicate to the string that it should expect to see a sequence that contains the values to be used by the string to populate its format specifiers Sequences are a very important part of programming in Python and they are covered in some detail later For now we are just going to use them What is important to know at this point is that every for mat specification in a string has to have an element that matches it in the sequence that s provided to it The items we a
13. re putting in the sequence are strings that are separated by commas if there is more than one If there is only one as in the preceding example the sequence isn t needed but it can be used The reason why this special escape sequence is called a format specifier is because you can do some other special things with it that is rather than just insert values you can provide some specifications about how the values will be presented how they ll look Try It Out More String Formatting You can do a couple of useful things when formatting a simple string gt gt gt Ss s 10s John Q Public John Q Public gt gt gt S 10s s 310s John Q Public John Os Public How It Works In the first string the reason why Public is so alone along the right side is because the third format specifier in the main string on the left side has been told to make room for something that has 10 char acters That s what the 310s means However because the word Public only has 6 characters Python padded the string with space for the remaining four characters that it had reserved Chapter 1 In the second string the Q is stranded in the middle with John and Public far to either side The behavior on its right hand side has just been explained The behavior on its left happens for very sim ilar reasons An area with 10 spaces has been created in the string but this string was specified with a 10s The in
14. t import wx gt gt gt from PythonCard import dialog util gt gt gt bg pcapp getCurrentBackground gt gt gt self bg gt gt gt comp bg components gt gt gt How It Works The codeEditor is a program written in Python and the Python shell within it is actually a special pro gramming environment that is enhanced with features that you will use later in the book to help you explore Python The import from and other statements are covered in Chapter 7 in depth but for now they re not important Beginning to Use Python Strings At this point you should feel free to experiment with using the shell s basic behavior Type some text in quotes for starters you could type the following gt gt gt This text really won t do anything This text really won t do anything gt gt gt You should notice one thing immediately After you entered a quote codeEditor s Python shell changed the color of everything up to the quote that completed the sentence Of course the preceding text is abso lutely true It did nothing It didn t change your Python environment it was merely evaluated by the run ning Python instance in case it did determine that in fact you d told it to do something In this case you ve asked it only to read the text you wrote but doing this doesn t constitute a change to the environment However you can see that Python indicated that it saw what you entered It showed you the
15. ters in some situations gt gt gt This is kind of a special string because it violates some 7 rules that we haven t talked about yet This is kind of a special string because it violates some n rules that we haven t talked about yet Chapter 1 As you can see here Python enables you to do what you want in triple quoted strings However it does raise one more question What s that n doing there In the text you created a new line by pressing the Enter key so why didn t it just print the rest of the sentence on another line Well Python will provide an interpretation to you in the interest of accuracy The reason why n may be more accurate than showing you the next character on a new line is twofold First that s one way for you to tell Python that you re interested in printing a new line so it s not a one way street Second when displaying this kind of data it can be confusing to actually be presented with a new line Without the n you may not know whether something is on a new line because you ve got a newline character or because there are spaces that lead up to the end of the line and the display you re using has wrapped around past the end of the current line and is continued on the next line By printing n Python shows you exactly what is happening Putting Two Strings Together Something that you are probably going to encounter more than a few times in your programming adventures is multiple strings that
16. text you entered and it displayed it in the manner it will always display a string in quotes As you learn about other data types you ll find that Python has a way of displaying each one differently What Is a String The string is the first data type that you re being introduced to within Python Computers in general and programming languages specifically segregate everything they deal with into types Types are cate gories for things within a program with which the program will work After a thing has a type the pro gram and the programmer knows what to do with that thing This is a fundamental aspect of how computers work because without a named type for the abstract ideas that they work with the computer won t know how to do basic things like combine two different values However if you have two things and they re of the same type you can define easy rules for combining them Therefore when the type of a thing has been confirmed Python knows what its options are and you as the programmer know more about what to do with it Chapter 1 Why the Quotes Now back to strings in particular Strings are the basic unit of text in Python Unlike some other pro gramming languages a single letter is represented as a one letter string Instead of trying to explain strings in terms of other concepts in a vacuum let s create some examples of strings using the Python shell and build from there Try It Out Entering Strings with
17. th num bers such as 38s to specify that you want space for eight characters no more and no less In later chapters you will learn about other format specifiers that work with other types You also learned how to print strings that you have created Printing is a type of input output operation input output is covered in more detail in Chapter 8 Using the print function you can present users of your program with strings that you have created In the next chapter you will learn about dealing with simple numbers and the operations that you can perform on them as well as how to combine numbers and strings so that print can render numbers dis playable This technique of using format specifiers will enable you to display other types of data as well Exercises 1 Inthe Python shell type the string Rock a by baby n ton the tree top t twhen the wind blows n t t t the cradle will drop Experiment with the number and placement of the t and n escape sequences and see how this affects what you see What do you think will happen 2 Inthe Python shell use the same string indicated in the prior exercise but display the string using the print function Again play with the number and location of the n and t escape sequences What do you think will happen 11
18. that specifier means that the item should be pushed to the left instead of to the right as it would normally Displaying Strings with Print Up until now you have seen how Python represents the strings you type but only how it represents them internally However you haven t actually done anything that your program would show to a user The point of the vast majority of programs is to present users with information programs produce everything from sports statistics to train schedules to web pages to automated telephone voice response units The key point is that they all have to make sense to a person eventually Try It Out Printing Text with Print For displaying text a special feature is built into useful languages one that helps the programmer dis play information to users The basic way to do this in Python is by using the print function gt gt gt print Ss s 10s John Q Public John Q Public gt gt gt You ll notice that there are no longer any quotes surrounding the first middle and last name In this case it s significant this is the first thing that you ve done that would actually be seen by someone using a program that you ve written How It Works print is a function a special name that you can put in your programs that will perform one or more tasks behind the scenes Normally you don t have to worry about how it happens When you start writ ing your own functions in Chapter 5 you
19. ve both Well there are a couple of reasons First strings play a huge part in almost any program that you re going to write and quotes define strings One challenge when you first use them is that quotes aren t special characters that appear only in computer programs They are a part of any normal English text to indicate that someone has spoken In addition they are used for emphasis or to indicate that something is literally what was seen or experienced The dilemma for a programming language is that when you re programming you can only use charac ters that are already on a keyboard However the keys on a keyboard can be entered by the average user so obviously people normally use those keys for tasks other than programming Therefore how do you make it a special character How do you indicate to the language that you the programmer mean some thing different when you type a set of quotes to pass a string to your program versus when you as the programmer enter quotes to explain something to the person using your program One solution to this dilemma is a technique that s called escaping In most programming languages at least one character called an escape character is designated and it has the power to remove the special Programming Basics and Strings significance from other special characters such as quotes This character in Python is the backslash Therefore if you have to quote some text within a string and it uses
20. ve occurred that prevented your program from working What All That Means Together Taken together these beginning principles mean that you re going to be introduced to programming as a way of telling a computer what tasks you want it to do in an environment where you are in control You will be aware that sometimes accidents can happen and that these mistakes can be accommodated through mechanisms that offer you some discretion regarding how these conditions will be handled including recovering from problems and continuing to work The First Steps First you should go online to the web site for the book following the procedure in the Introduction and follow the instructions there for downloading PythonCard PythonCard is a set of utilities that provides an environment for programming in Python PythonCard is a product that s free to use and distribute and is tailor made for writing in Python It contains an editor called codeEditor that you will be using for the first part of this book It has a lot in common with the editor that comes with Python called idle but in the opinion of the authors codeEditor works better as a teaching tool because it was written with a focus on users who may be working on simpler projects In addition codeEditor is a program written in Python Programs are written in a form called source code Source code contains the instruc tions that the language follows and when the source code is read and processe
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