Python Essentials 1: Module 2 Data types, variables, basic input-output operations, basic operators.  In this module, you will learn:  How to write an run simple Python programs what Python literals, operators, and expressions are what variables are and what are the rules that govern them how to perform basic input and output operations

Hello, World It's time to start writing some real, working Python code. It'll be very simple for the time being.  As we're going to show you some fundamental concepts and terms, these snippets of code won't be serious or complex.  Run the code in the editor window on the right. If everything goes okay here, you'll see the line of text in the console window.  Alternatively, launch IDLE, create a new Python source file, fill it with this code , name the file and save it. Now run it. If everything goes okay, you'll see the rhyme's line in the IDLE console window. The code you have run should look familiar. You saw something very similar when we led you through the setting up of the IDLE environment.  Now we'll spend some time showing and explaining to you what you're actually seeing, and why it looks like this.  As you can see, the first program consists of the following parts:  the word print an opening parenthesis a quotation mark a line of text: Hello, World! another quotation mark a closing parenthesis Each of the above plays a very important role in the code

The print() function Look at the line of code below: print ("Hello, World!") The word print that you can see here is a function name. That doesn't mean that wherever the word appears it is always a function name. The meaning of the word comes from the context in which the word has been used.  You've probably encountered the term function many times before, during math classes. You can probably also list several names of mathematical functions, line sine or log.  Python functions, however, are more flexible, and can contain more content than their mathematical siblings.  A function (in this context) is a separate part of the computer code able to:  cause some effect (e.g., send text to the terminal, create a file, draw an image, play a sound, etc.); this is something completely unheard of in the world of mathematics.  evaluate a value (e.g., the square root of a value or the length of a given text) and return it as the function's result; this is what makes Python functions the relatives of mathematical concepts.  Morever, many of Python functions can do the above two things together.  Where do functions come from?  They may come from Python itself; the print function is one of this kind; suhc a function is an added value received together with Python and its environment (it is built-in); you don't have to do anything special (e.g., ask anyone for anything) if you want to make use of it they may come from one or more of Python's add-ons name modules; some of the modules come with Python, others may require separate installation - whatever the case, they all need to be explicitily connected with your code (we'll show you how to do that soon) You can  write them yourself, placing as many functions as you want and need inside your program to make it simpler, cleare and more elegant The name of the function should be significant (the name of the print function is self-evident) Of course, if you're going to make sure of any already existing function, you have no influence on its name, but when you start writing your own functions, you should considet your choice of names.   

The print () function As we said before, a function may have:  an effect a result There's also a third, very important, function component - the argument(s). Mathematical functions usually take one argument, e.g., sin(x) takes an x, which is the measure of an angle.  Python functions, on the other hand, are more versatile. Depending on the individual needs, they may accept any number of arguments - as many as necessary to perform their tasks. Note: any number includes - zero -some Python functions don't need any argument print("Hello, World!") In spite of the number of needed/provided arguments, Python functions strongly demand the presence of a pair of parentheses - opening and closing ones respectively.  If you want to deliver one of more arguments to a function, you place them inside the parentheses. If you're going to use a function which doesn't take any argument, you still have to have the parentheses.  Note: to distinguish ordinary words from function names, place a pair of empty parentheses after their names, even if corresponding function wants one or more arguments. This is a standard convention.  The function we're talking about here is print() Does the print() function in our example have any arguments?  Of course it does, but what are they? 

The print() function The only argument delivered to the print() function in this example is a string: print("Hello, World!") As you can see, the string is delimited with quotes - in fact, the quotes make the string - they cut out a part of the code and assign a different meaning to it.  You can imagine that the quotes say something like: the text between us is not code It isn't intended to be executed, and you should take it as is.  Almost anything you put inside the quotes will be take literally, not as code but as data. Try to play with this particular string, - modify it, enter some new content, delete some of the existing content.  There's more than one way to specify a string inside Python's code, but for now, though, this one is enough.  So far, you have learned about two important parts of the code: the function and the string. We've talked about them in terms of syntax, but now it's time to discuss them in terms of semantics. 

The print() function The function name (print in this case) along with the parentheses and argument(s), forms the function invocation.  We'll discuss this in more depth soon, but we should just shed a little light on it right now.  print("Hello, World!") What happens when Python encounters an invocation like this one below?  function_name(argument) Let's see: First, Python Checks if the name specified is legal (it browses its internal data in order to find an existing function of the name; if this search fails, Python aborts the code) Second, Python checks if the function's requirements for the number of arguments allows you to invoke the function in this way (e.g., if a specific function demands exactly two arguments, any invocation delivering only one argument will be considered erroneous, and will abort the code's execution) Third, Python leaves your coe for a moment and jumps into the function you want to invoke; of course, it takes your argument(s) too and passes it/them to the function; Fourth, the function executes its code, causes the desired effect (if any), evaluates the desired result(s) (if any) and finishes its task Finally, Python returns to your code (to the place just after the invocation) and resumes its execution  

LAB Estimated time 5-10 minutes Level of difficulty Very Easy Objectives becoming familiar with the print() function and its formatting capabilities experimenting with Python code. Scenario The print() command, which is one of the easiest directives in Python, simply prints out a line to the screen.  In your first lab: use the print() function to print the line Hello, Python! to the screen. use double quotes around the string having done that, use the print() function again, but this time print your First name; remove the double quotes and run your code. Watch Python's reaction. What kind of error is thrown?  SyntaxError: invalid syntax then, remove the parentheses, put back the double quotes, - and run your code again. What kind of error is thrown this time?  No Error, Hello, Q David Khieu experiment as much as you can. Change double quotes to single quotes, use multiple print() functions on different lines. See what happens. 

The print() function Three important questions have to be answered as soon as possible: What is the effect the print() function causes? The effect is very useful and very spectacular. The function: takes its arguments (it may accept more than one argument and may also accept less than one argument) converts them into human-readable form if needed (as you may suspect, strings don't require this action, as the string is already readable) and sends the resulting data to the output device (usually the console); in other words, anything you put into the print() function will appear on your screen.              No wonder then, that from now on, you'll utilize print() very intensively to see the results of your operations and evaluations.        2. What arguments does print() expect?             Any. We'll show you soon that print() is able to operate with virtually all types of data offered by Python. Strings, Numbers, characters, logical values, objects - any of these may be successfully passed to print().        3. What value does the print() function return?             None its effect is enough. 

The print() function- instructions You have already seen a computer program that contains one function invocation. A function invocation is one of many possible kinds of Python instructions.  Of course, any complex program usually contains many more instructions than one. The question is: how do you couple more than one instruction into the Python code?  Python's syntax is quite specific in this area. Unlike most programming languages, Python requires that there cannot be more than one instruction in a line.  A line can be empty (i.e., it may contain no instruction at all) but it must not contain two, three or more instructions. This is strictly prohibited.  Note: Python makes one exception to this rule - it allows one instruction to spread across more than one line )which may be helpful when your code contains complex constructions).  Let's expand the code a bit, you can see it in the editor. Run it and note what you see in the console.  Your Python console should now look like this:  This is a good opportunity to make some observations: the program invokes the print() function twice, and you can see two separate lines in the console - this means that print() begins its output from a new line each time it starts its execution; you can change this behavior, but you can also use it to your advantage; each print() invocation contains a different string, as its argument and the console content reflects it - this means that the instructions in the code are executed in the same order in which they have been place in the source file; no next instruction is executed until the previous one is completed (there are some exceptions to this rule, but you can ignore them for now)   

The print() function - instructions We've changed the example a bit - we've added one empty print() function invocation. We call it empty because we haven't delivered any arguments to the function.  You can see it in the editor window. Run the code.  What happens?  If everything goes right, you should see something like this:  As you can see, the empty print() invocation is not as empty as you may have expected - it does output an empty line, or (this interpretation is also correct) its output is just a newline.  This is not the only way to produce a newline in the output console. We're now going to show you another way. 

The print() function - the escape and newline characters We've modified the code again. Look at it carefully.  There are two very subtle changes - we've inserted a strange pair of characters inside the rhyme. They look like this: \n.  Interestingly, while you can see two characters, Python sees one.  the backslash (\_ has a very special meaning when used inside strings - this is called the escape character. The word escape should be understood specifically - it means that the series of characters in the string escapes for the moment (a very short moment) to introduce a special inclusion.  In other words, the backslash doesn't mean anything in itself, but is only a kind of announcement, that the next character after the backslash has a different meaning too.  The letter n placed after the backslash comes from the word newline. Both the backslash and the n form a special symbol named a newline character, which urges the console to start a new output line Run the code. Your console should now look like this:  As you can see, two newlines appear in the nursery rhyme, the places where the \n have been used. 

The print() function - the escape and newline characters This convention has two important consequences: If you want to put just one backslash inside a string, don't forget its escaping nature - you have to double it, e.g., such an invocation will cause an error: print("\") while this one won't" pritn("\\") Not all escape pairs (the backslash coupled with another character) mean something.  Experiment with your code in the editor, run it and see what happens. 

The print() function - using multiple arguments So far we have teste the print()  function behavior with no arguments, and with one argument. It's also worth trying to feed the print() function with more than one argument.  Look at the editor window. This is what we're going to test now.  There is one print() function invocation, but it contains three arguments. All of them are strings.  The arguments are separated by commas. We've surrounded them with spaces to make them more visible, but it's not really necessary, and we won't be doing it anymore. In this case, the commas separating the arguments play a completely different role than the comma inside the string. The former is a part of Python's syntax, the latter is intended to be shown in the console.  If you look at the code again, you'll see that there are no spaces inside the strings.  Run the code and see what happens.  The console should now be showing the following text.  The itsy bitsy spider climbed up the waterspout. The spaces, removed from the strings; have appeared again. Can you explain why?  Two conclusions emerge from this example:  a print() function invoked with more than one argument outputs them all on one line; the print() function puts a space between the outputted arguments on its own initiative.

The print() function - the positional way of passing the arguments Now that you know a bit about print() function customs, we're going to show you how to change them.  You should be able to predict the output without running the code in the editor.  The way in which we are passing the arguments into the print() function is the most common in Python, and is called the positional way (this name comes from the fact that the meaning of the argument is dictated by its position, e.g., the second argument will be outputted after the first, not the other way round) Run the code and check if the output matches your preditions.   

The print() function - the keyword arguments Python offers another mechanism for the passing of arguments, which can be helpful when you want to convince the print() function to change its behaviour a bit.  We aren't going to explain it in depth right now. We plan to do this when we talk about functions. For now we simply want to show you how it works. Feel free to use it in your own programs.  The mechanism is called keyword arguments.  The name stems from the fact the meaning of these arguments is taken not from its location (position) but from the special word (keyword) used to identify them.  The print() function has two keyword arguments that you can use for your purposes. The first of them is named end In the editor window you can see a very simple example of using a keyword argument.  In order to use it, it is necessary to know some rules:  a keyword argument consists of three elements: a keyword identifying the argument (end here); an  equal sign (=); and a value  assigned to that argument; any keyword arguments have to be put after the last positional argument (this is very important) In our example, we have made use of the end keyword argument, and set it to a string containing one space.  Run the code to see how it works.  The console should now be showing the following text:  My name is Python. Monty Python. As you can see, the end keyword argument determines the characters the print() function sends to the output once it reaches the end of its positional arguments. The default behaviour reflects the situation where the end keyword argument is implicitly used in the following way: end="\n"  

The print() function - the keyword arguments And now it's time to try something more difficult.  If you look carefully, you'll se that we've used the end argument, but the string assigned to it is empty (it contains no characters at all). What will happen now? Run the program in the editor to find out.  As the end argument has been set to nothing, the print() function outputs nothing too, once its positional arguments have been exhausted.  The console should now be showing the following text:  My name is Monty Python. Note: no newlines have been sent to the output The string assigned to the end keyword argument an be of any length. Experiment if you want. 

The print() function - the keyword arguments We've said previously that the print() function separates its outputted arguments with spaces. This behaviour can be changed, too.  The keyword argument that can do this is named sep (like separator) Look at the code in the editor and run it.  The sep argument delivers the following results:  My-name-is-Monty-Python. The print() function now uses a dash, instead of a space, to separate the outputted arguments.  Note: the sep argument's value may be an empty string, too. Try it for yourself.   

The print() function - the keyword arguments Both keyword arguments may be mixed in one invocation, just like here in the editor window.  The example doesn't make much sense, but it visibly presents the interactions between end and sep.  Can you predict the output?  Run the code and see if it matches your predictions.  Now that you understand the print() function, you're ready to consider how to store and process data in Python.  Without print() , you wouldn't be able to see any results. 

LAB Estimated time 5-10 minutes Level of difficulty Very Easy Objectives becoming familiar with the print() function and it's formatting capabilities experimenting with Python code Scenario Modify the first line of code in the editor, using the sep and the end keywords, to match the expected output. Use the two print() functions in the editor.  Don't change anything in the second print() invocation. 

Key takeaways The print() function is a built-in function. It prints/outputs a specified message to the screen/console window Built-in functions, contrary to user-defined functions, are always available and don't have to be imported. Python 3.8 comes with 69 built-in functions. You can find their full list provided in alphabetical order in the Python Standard Library To call a function (this process is known as function invocation or function call), You need to use the function name followed by parentheses. You can pass arguments into function by placing them inside the parentheses. You can pass arguments into a function by placing them inside the parentheses. You must separate arguments with a comma, e.g., print ("Hello,", "world!"). an "empty" print() function outputs an empty line to the screen.  Python strings are delimited with quotes, e.g., "I am a string" (double quotes), or 'I am a string, too' (single quotes) Computer programs are collections of instructions. An instruction is a command to perform a specific task when executed. e.g, to print a certain message to the screen.  In Python strings the backslash (\) is a special character which announces that the next character has a different meaning, e.g. \n (the newline character) starts a new output line.  Positional arguments are the ones whose meaning is dictated by their position, e.g., the second argument is outputted after the first, the third is outputted after the second, etc.  The end and sep parameteres can be used for formatting the output of the print() function. The sep parameter specifies the separator between the outputted arguments (e.g., print ('H", "E", "L","L", "O", sep="-") wherease the end paremeter specifies what to print at the end of the print statement.