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We will learn about the powerful capabilities of Python and how to compose those capabilities together to create useful programs. There are some low-level conceptual patterns that we use to construct programs. These constructs are not just for Python programs, they are part of every programming language from machine l...
This might be reading data from a** file, or even some kind of sensor like a microphone or GPS.
In our initial programs, our input will come from the user typing data on the keyboard. output Display the results of the program on a screen or store them in a file or perhaps write them to a device like a speaker to play music or speak text. **sequential execution Perform statements one after another in the ord...
It is like saying that walking is simply “putting one foot in front of the other”.
The “art” of writing a program is composing and weaving these basic elements together many times over to produce something that is useful to its users. The word counting program above directly uses all of these patterns except for one. ##### 1.10 What could possibly go wrong? As we saw in our earliest conversations ...
The smallest deviation or mistake will cause Python to give up looking at your program. Beginning programmers often take the fact that Python leaves no room for errors as evidence that Python is mean, hateful, and cruel.
While Python seems to like everyone else, Python knows them personally and holds a grudge against them. Because of this grudge, Python takes our perfectly written programs and rejects them as “unfit” just to torment us. >>> primt 'Hello world!' File "<stdin>", line 1 primt 'Hello world!' ^ SyntaxError: invalid syn...
It is just a tool. It has no emotions and it is happy and ready to serve you whenever you need it. Its error messages sound harsh, but they are just Python’s call for help.
It has looked at what you typed, and it simply cannot understand what you have entered. Python is much more like a dog, loving you unconditionally, having a few key words that it understands, looking you with a sweet look on its face (>>>), and waiting for you to say something it understands.
When Python says “SyntaxError: invalid syntax”, it is simply wagging its tail and saying, “You seemed to say something but I just don’t understand what you meant, but please keep talking to me (>>>).” As your programs become increasingly sophisticated, you will encounter three general types of errors: **Syntax errors...
A** syntax error means that you have violated the “grammar” rules of Python. Python does its best to point right at the line and character where it noticed it was confused.
The only tricky bit of syntax errors is that sometimes the mistake that needs fixing is actually earlier in the program than where Python _noticed it was confused.
So the line and character that Python indicates in_ a syntax error may just be a starting point for your investigation. Logic errors A logic error is when your program has good syntax but there is a mistake in the order of the statements or perhaps a mistake in how the statements relate to one another.
A good example of a logic error might be, “take a drink from your water bottle, put it in your backpack, walk to the library, and then put the top back on the bottle.” Semantic errors A semantic error is when your description of the steps to take is syntactically perfect and in the right order, but there is simply...
The program is perfectly correct but it does not do what you _intended for it to do. A simple example would be if you were giving a person_ directions to a restaurant and said, “. . .
when you reach the intersection with the gas station, turn left and go one mile and the restaurant is a red building on your left.” Your friend is very late and calls you to tell you that they are on a farm and walking around behind a barn, with no sign of a restaurant. Then you say “did you turn left or right at the g...
When you were learning to speak, it was not a problem for your first few years that you just made cute gurgling noises. And it was OK if it took six months for you to move from simple vocabulary to simple sentences and took 5-6 more years to move from sentences to paragraphs, and a few more years to be able to write an...
But it is like learning a new language that takes time to absorb and understand before it feels natural.
That leads to some confusion as we visit and revisit topics to try to get you to see the big picture while we are defining the tiny fragments that make up that big picture.
While the book is written linearly, and if you are taking a course it will progress in a linear fashion, don’t hesitate to be very nonlinear in how you approach the material.
Look forwards and backwards and read with a light touch. By skimming more advanced material without fully understanding the details, you can get a better understanding of the “why?” of programming.
By reviewing previous material and even redoing earlier exercises, you will realize that you actually learned a lot of material even if the material you are currently staring at seems a bit impenetrable. Usually when you are learning your first programming language, there are a few wonderful “Ah Hah!” moments where yo...
Take a break, take a nap, have a snack, explain what you are having a problem with to someone (or perhaps your dog), and then come back to it with fresh eyes.
I assure you that once you learn the programming concepts in the book you will look back and see that it was all really easy and elegant and it simply took you a bit of time to absorb it. ##### 1.12 Glossary bug An error in a program. **central processing unit The heart of any computer.
It is what runs the software that we write; also called “CPU” or “the processor”. compile To translate a program written in a high-level language into a low-level language all at once, in preparation for later execution. ----- high-level language A programming language like Python that is designed to** be e...
Main memory loses its information when the power is turned off. parse To examine a program and analyze the syntactic structure. portability A property of a program that can run on more than one kind of computer. print function An instruction that causes the Python interpreter to display a** value on the ...
Generally slower than main memory.
Examples of secondary memory include disk drives and flash memory in USB sticks. semantics The meaning of a program. semantic error An error in a program that makes it do something other than what the programmer intended. source code A program in a high-level language. ----- ##### 1.13 Exercises Exer...
For example, “What is the human equivalent to a Central Processing Unit”? Exercise 9: How do you fix a “Syntax Error”? ----- ----- ## Chapter 2 # Variables, expressions, and statements ##### 2.1 Values and types A value is one of the basic things a program works with, like a letter or a number. The values we ha...
You (and the interpreter)_ can identify strings because they are enclosed in quotation marks. The print statement also works for integers.
We use the python command to start the interpreter. python >>> print(4) 4 If you are not sure what type a value has, the interpreter can tell you. >>> type('Hello, World!') <class 'str'> >>> type(17) <class 'int'> Not surprisingly, strings belong to the type str and integers belong to the type int.
Less obviously, numbers with a decimal point belong to a type called float, because these numbers are represented in a format called floating point. >>> type(3.2) <class 'float'> What about values like “17” and “3.2”?
They look like numbers, but they are in quotation marks like strings. ----- >>> type('17') <class 'str'> >>> type('3.2') <class 'str'> They’re strings. When you type a large integer, you might be tempted to use commas between groups of three digits, as in 1,000,000.
This is not a legal integer in Python, but it is legal: >>> print(1,000,000) 1 0 0 Well, that’s not what we expected at all!
Python interprets 1,000,000 as a commaseparated sequence of integers, which it prints with spaces between. This is the first example we have seen of a semantic error: the code runs without producing an error message, but it doesn’t do the “right” thing. ##### 2.2 Variables One of the most powerful features of a prog...
A variable is a name that refers to a value. An assignment statement creates new variables and gives them values: >>> message = 'And now for something completely different' >>> n = 17 >>> pi = 3.1415926535897931 This example makes three assignments.
The first assigns a string to a new variable named message; the second assigns the integer 17 to n; the third assigns the (approximate) value of π to pi. To display the value of a variable, you can use a print statement: >>> print(n) 17 >>> print(pi) 3.141592653589793 The type of a variable is the type of the value ...
They can contain both letters and numbers, but they cannot start with a number.
It is legal to use uppercase letters, but it is a good idea to begin variable names with a lowercase letter (you’ll see why later). The underscore character (_) can appear in a name.
It is often used in names with multiple words, such as my_name or airspeed_of_unladen_swallow.
Variable names can start with an underscore character, but we generally avoid doing this unless we are writing library code for others to use. If you give a variable an illegal name, you get a syntax error: >>> 76trombones = 'big parade' SyntaxError: invalid syntax >>> more@ = 1000000 SyntaxError: invalid syntax >>> ...
more@ is illegal because it contains an illegal character, @. But what’s wrong with class? It turns out that class is one of Python’s keywords.
The interpreter uses keywords to recognize the structure of the program, and they cannot be used as variable names. Python reserves 33 keywords: and del from None True as elif global nonlocal try assert else if not while break except import or with class False in pass yield continue finally is raise def for lambda re...
If the interpreter complains about one of your variable names and you don’t know why, see if it is on this list. ##### 2.4 Statements A statement is a unit of code that the Python interpreter can execute.
We have seen two kinds of statements: print being an expression statement and assignment. When you type a statement in interactive mode, the interpreter executes it and displays the result, if there is one. ----- A script usually contains a sequence of statements.
If there is more than one statement, the results appear one at a time as the statements execute. For example, the script print(1) x = 2 print(x) produces the output 1 2 The assignment statement produces no output. ##### 2.5 Operators and operands _Operators are special symbols that represent computations like ad...
The values the operator is applied to are called operands. The operators +, -, , /, and * perform addition, subtraction, multiplication, division, and exponentiation, as in the following examples: 20+32 hour-1 hour60+minute minute/60 52 (5+9)(15-7) There has been a change in the division operator between Pytho...
In Python 3.x, the result of this division is a floating point result: >>> minute = 59 >>> minute/60 0.9833333333333333 The division operator in Python 2.0 would divide two integers and truncate the result to an integer: >>> minute = 59 >>> minute/60 0 To obtain the same answer in Python 3.0 use floored ( // intege...
A value all by itself is considered an expression, and so is a variable, so the following are all legal expressions (assuming that the variable x has been assigned a value): 17 x x + 17 If you type an expression in interactive mode, the interpreter evaluates it and displays the result: >>> 1 + 1 2 But in a script, ...
This is a common source of confusion for beginners. Exercise 1: Type the following statements in the Python interpreter to see what they do: 5 x = 5 x + 1 ##### 2.7 Order of operations When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.
For mathematical operators, Python follows mathematical convention.
The acronym PEMDAS is a useful way to remember the rules: - Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want.
Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8.
You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even if it doesn’t change the result. - Exponentiation has the next highest precedence, so 2*1+1 is 3, not 4, and 31**3 is 3, not 27. - M ultiplication and Division have the same precedence, which is higher than _Additi...
So 2*3-1_ is 5, not 4, and 6+4/2 is 8.0, not 5. - Operators with the same precedence are evaluated from left to right.
So the expression 5-3-1 is 1, not 3, because the 5-3 happens first and then 1 is subtracted from 2. When in doubt, always put parentheses in your expressions to make sure the computations are performed in the order you intend. ----- ##### 2.8 Modulus operator The modulus operator works on integers and yields the r...
In Python, the modulus operator is a percent sign (%).
The syntax is the same as for other operators: >>> quotient = 7 // 3 >>> print(quotient) 2 >>> remainder = 7 % 3 >>> print(remainder) 1 So 7 divided by 3 is 2 with 1 left over. The modulus operator turns out to be surprisingly useful.
For example, you can check whether one number is divisible by another: if x % y is zero, then x is divisible by y. You can also extract the right-most digit or digits from a number.
For example, x % 10 yields the right-most digit of x (in base 10).
Similarly, x % 100 yields the last two digits. ##### 2.9 String operations The + operator works with strings, but it is not addition in the mathematical sense. Instead it performs concatenation, which means joining the strings by linking them end to end.
For example: >>> first = 10 >>> second = 15 >>> print(first+second) 25 >>> first = '100' >>> second = '150' >>> print(first + second) 100150 The output of this program is 100150. ##### 2.10 Asking the user for input Sometimes we would like to take the value for a variable from the user via their keyboard.
Python provides a built-in function called input that gets input from the keyboard[1]. When this function is called, the program stops and waits for the user to type something.
When the user presses Return or Enter, the program resumes and input returns what the user typed as a string. 1In Python 2.0, this function was named raw_input. ----- >>> input = input() Some silly stuff >>> print(input) Some silly stuff Before getting input from the user, it is a good idea to print a prompt telli...
You can pass a string to input to be displayed to the user before pausing for input: >>> name = input('What is your name?\n') What is your name? Chuck >>> print(name) Chuck The sequence \n at the end of the prompt represents a newline, which is a special character that causes a line break.
That’s why the user’s input appears below the prompt. If you expect the user to type an integer, you can try to convert the return value to int using the int() function: >>> prompt = 'What...is the airspeed velocity of an unladen swallow?\n' >>> speed = input(prompt) What...is the airspeed velocity of an unladen swal...
These notes are called comments, and in Python they start with the # symbol: ----- _# compute the percentage of the hour that has elapsed_ percentage = (minute * 100) / 60 In this case, the comment appears on a line by itself.
You can also put comments at the end of a line: percentage = (minute * 100) / 60 _# percentage of an hour_ Everything from the \# to the end of the line is ignored; it has no effect on the program. Comments are most useful when they document non-obvious features of the code. It is reasonable to assume that the reade...
In the beginning, this choice can be confusing both when you read a program and when you write your own programs.
For example, the following three programs are identical in terms of what they accomplish, but very different when you read them and try to understand them. a = 35.0 b = 12.50 c = a * b print(c) hours = 35.0 rate = 12.50 pay = hours * rate print(pay) x1q3z9ahd = 35.0 x1q3z9afd = 12.50 x1q3p9afd = x1q3z9ahd * x1q3z9af...
Humans will most quickly understand the intent of the second program because the programmer has chosen variable names that reflect their intent regarding what data will be stored in each variable. We call these wisely chosen variable names “mnemonic variable names”.
The word _mnemonic[2]_ means “memory aid”.
We choose mnemonic variable names to help us remember why we created the variable in the first place. While this all sounds great, and it is a very good idea to use mnemonic variable names, mnemonic variable names can get in the way of a beginning programmer’s ability to parse and understand code.
This is because beginning programmers have not yet memorized the reserved words (there are only 31 of them) and sometimes variables with names that are too descriptive start to look like part of the language and not just well-chosen variable names. Take a quick look at the following Python sample code which loops thro...
We will cover loops soon, but for now try to just puzzle through what this means: for word in words: print(word) What is happening here?
Which of the tokens (for, word, in, etc.) are reserved words and which are just variable names? Does Python understand at a fundamental level the notion of words?
Beginning programmers have trouble separating what parts of the code must be the same as this example and what parts of the code are simply choices made by the programmer. The following code is equivalent to the above code: for slice in pizza: print(slice) It is easier for the beginning programmer to look at th...
It is pretty clear that Python has no fundamental understanding of pizza and slices and the fact that a pizza consists of a set of one or more slices. But if our program is truly about reading data and looking for words in the data, pizza and slice are very un-mnemonic variable names.
Choosing them as variable names distracts from the meaning of the program. After a pretty short period of time, you will know the most common reserved words and you will start to see the reserved words jumping out at you: word in words:\ print word The parts of the code that are defined by Python (for, in, pri...
Many [2See http://en.wikipedia.org/wiki/Mnemonic for an extended description of the word](http://en.wikipedia.org/wiki/Mnemonic) “mnemonic”. ----- text editors are aware of Python syntax and will color reserved words differently to give you clues to keep your variables and reserved words separate.
After a while you will begin to read Python and quickly determine what is a variable and what is a reserved word. ##### 2.13 Debugging At this point, the syntax error you are most likely to make is an illegal variable name, like class and yield, which are keywords, or odd~job and US$, which contain illegal characters...
The most common messages are SyntaxError: invalid syntax and SyntaxError: invalid token, neither of which is very informative. The runtime error you are most likely to make is a “use before def;” that is, trying to use a variable before you have assigned a value.
This can happen if you spell a variable name wrong: >>> principal = 327.68 >>> interest = principle * rate NameError: name 'principle' is not defined Variables names are case sensitive, so LaTeX is not the same as latex. At this point, the most likely cause of a semantic error is the order of operations. For example...
We often give variables mnemonic names to help us remember what is stored in the variable. modulus operator An operator, denoted with a percent sign (%), that works on integers and yields the remainder when one number is divided by another. operand One of the values on which an operator operates. operato...
So far, the statements we have seen are assignments and print expression statement. string A type that represents sequences of characters. type A category of values.
The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str). value One of the basic units of data, like a number or string, that a program manipulates. variable A name that refers to a value.** ##### 2.15 Exercises Exercise 2: Write a program that uses ...
If you want, you can play with the built-in Python round function to properly round the resulting pay to two decimal places. Exercise 4: Assume that we execute the following assignment statements: width = 17 height = 12.0 For each of the following expressions, write the value of the expression and the type (of the v...
width//2 2. width/2.0 3. height/3 4.
1 + 2 \* 5 Use the Python interpreter to check your answers. Exercise 5: Write a program which prompts the user for a Celsius temperature, convert the temperature to Fahrenheit, and print out the converted temperature. ----- ## Chapter 3 # Conditional execution ##### 3.1 Boolean expressions A boolean expression...
The following examples use the operator ==, which compares two operands and produces True if they are equal and False otherwise: >>> 5 == 5 True >>> 5 == 6 False {} True and False are special values that belong to the class bool; they are not strings: >>> type(True) <class 'bool'> >>> type(False) <class 'bool'> The...
A common error is to use a single equal sign (=) instead of a double equal sign (==). Remember that = is an assignment operator and == is a comparison operator.
There is no such thing as =< or =>. ----- ##### 3.2 Logical operators There are three logical operators: and, or, and not.
The semantics (meaning) of these operators is similar to their meaning in English.
For example, x > 0 and x < 10 is true only if x is greater than 0 and less than 10. n%2 == 0 or n%3 == 0 is true if either of the conditions is true, that is, if the number is divisible by 2 or 3. Finally, the not operator negates a boolean expression, so not (x > y) is true if x > y is false; that is, if x is less...
Any nonzero number is interpreted as “true.” >>> 17 and True True This flexibility can be useful, but there are some subtleties to it that might be confusing.
You might want to avoid it until you are sure you know what you are doing. ##### 3.3 Conditional execution In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly.
Conditional statements give us this ability. The simplest form is the if statement: if x > 0 : print('x is positive') The boolean expression after the if statement is called the condition.
We end the if statement with a colon character (:) and the line(s) after the if statement are indented. x > 0 Figure 3.1: If Logic ----- If the logical condition is true, then the indented statement gets executed.
If the logical condition is false, the indented statement is skipped. if statements have the same structure as function definitions or for loops[1].
The statement consists of a header line that ends with the colon character (:) followed by an indented block.

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