Python basics

Introduction

In this chapter, we introduce the basics of the Python programming language. At this point you should already have set up a development environment for writing and running your Python code. It will be assumed in the text that this is the case. If you are having trouble setting up Python, contact a teaching assistant or post a message to the course forum. Throughout this course, you are strongly encouraged to try what you have learnt by writing an actual program. You can only learn how to program by actually doing it yourself.
Each chapter contains several exercises to help you practice. Solutions are found at the end of the chapter.

Python 2 vs Python 3

Python recently underwent a major version change from 2 to 3. For consistency with other courses in the department, we will be using Python 3. Python 2 is still widely used, and although Python 3 is not fully backwards compatible the two versions are very similar – so should you ever encounter Python 2 code you should find it quite familiar. We will mention important differences between the two versions throughout this course. We will always refer to the latest version of Python 2, which at the time of writing was 2.7.

Getting started with Python

Using the interactive interpreter

Entering python on the commandline without any parameters will launch the Python interpreter. This is a text console in which you can enter Python commands one by one – they will be interpreted on the fly.
Note
In these notes we will assume throughout that the python command launches Python 3, but if you have both Python 2 and Python 3 installed on your computer, you may need to specify that you want to use Python 3 by using the python3 command instead. Whenever you launch the interpreter, you will see some information printed before the prompt, which includes the version number – make sure that it starts with 3! Take note of the command that you need to use.
Here is an example of an interpreter prompt:
Python 3.2.3 (default, Oct 19 2012, 20:10:41)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
If you type a number, string or any variable into the interpreter, its value will automatically be echoed to the console:
>>> "hello"
'hello'
>>> 3
3
That means that you don’t have to use an explicit print command to display the value of a variable if you are using the interpreter – you can just enter the bare variable, like this:
>>> x = 2
>>> x
2
This won’t work if you are running a program from a file – if you were to enter the two lines above into a file and run it, you wouldn’t see any output at all. You would have to use the print function to output the value of x:
x = 2
print(x)
In most of the code examples in this module we have used explicit print statements, so that you will see the same output whether you use the examples in the interpreter or run them from files.
The interpreter can be very useful when you want to test out a small piece of code before adding it to a larger program. It’s a quick and easy way to check how a function works or make sure that the syntax of a code fragment is correct.
There are some other interactive interpreters for Python which have more advanced features than the built-in interpreter, for example functionality for inspecting the contents of objects or querying the documentation for imported modules, classes and functions:
  • IPython, which was originally developed within the scientific community
  • bpython, a new project

Running programs from files

The interpreter is useful for testing code snippets and exploring functions and modules, but to save a program permanently we need to write it into a file. Python files are commonly given the suffix .py. Once you have written a program and saved it, you can run it by using the python command with the file name as a parameter:
python myprogram.py
This will cause Python to execute the program.
Like any source code file, a Python file is just an ordinary text file. You can edit it with any text editor you like. It is a good idea to use a text editor which at least supports syntax highlighting – that is, it can display the words in your program in different colours, depending on the function they perform in your program. It is also useful to have indentation features such as the ability to indent or unindent blocks of code all at once, and automatic indentation (having the program guess the right level of indentation whenever you start typing a new line).
Some programmers prefer to use an integrated development environment, or IDE. An IDE is a program which combines a text editor with additional functionality like looking up documentation, inspecting objects, compiling the code (in the case of a compiled language) and running the code. Some IDEs support multiple languages, and some are designed for a specific language.
There are many IDEs, free and commercial, which can be used with Python. Python also comes with a simple built-in IDE called IDLE (you may need to install it from a separate package).

Installing new packages

How you install new Python packages depends a little on your operating system. Linux distributions have their own package managers, and you may choose to install packages using these managers so that they are integrated with the other packages on your system. However, some obscure Python packages may not be available as system packages, and the packages which are available are often not the latest versions. It is thus sometimes necessary to install packages directly from PyPI.
The Python Package Index (PyPI) is a large repository of Python packages. You can install packages from this repository using a tool like easy_install or pip (which is intended to be a more modern replacement for easy_install). Both of these utilities are cross-platform. Here is how you install a package called sqlobject with pip:
pip install sqlobject
This command will search PyPI for a package called sqlobject, download it and install it on your system.

Further reading

In this module we will see many examples of Python’s built-in functions and types and modules in the standard library – but this document is only a summary, and not an exhaustive list of all the features of the language. As you work on the exercises in this module, you should use the official Python documentation as a reference.
For example, each module in the standard library has a section in the documentation which describes its application programming interface, or API – the functionality which is available to you when you use the module in your code. By looking up the API you will be able to see what functions the module provides, what input they require, what output they return, and so on. The documentation often includes helpful examples which show you how the module is meant to be used.
The documentation is available on the web, but you can also install it on your computer – you can either download a copy of the documentation files in HTML format so that you can browse them locally, or use a tool like pydoc, which prints out the documentation on the commandline:
pydoc re

Essentials of a Python program

In most of today’s written languages, words by themselves do not make sense unless they are in certain order and surrounded by correct punctuation symbols. This is also the case with the Python programming language. The Python interpreter is able to interpret and run correctly structured Python programs. For example, the following Python code is correctly structured and will run:
print("Hello, world!")
Many other languages require a lot more structure in their simplest programs, but in Python this single line, which prints a short message, is sufficient. It is not, however, a very informative example of Python’s syntax – so here is a slightly more complex program which does (almost) exactly the same thing:
# Here is the main function.
def my_function():
    print("Hello, World!")

my_function()
This type of program is often referred to as a skeleton program, because one can build upon it to create a more complex program.
Note
The first line of the skeleton program is a comment. A hash (#) denotes the start of a comment. The interpreter will ignore everything that follows the hash until the end of the line. Comments will be discussed further in the later part of this unit.

Keywords

In the code above, the words def and if are keywords or reserved words, i.e. they have been kept for specific purposes and may not be used for any other purposes in the program. The following are keywords in Python:
False      class      finally    is         return
None       continue   for        lambda     try
True       def        from       nonlocal   while
and        del        global     not        with
as         elif       if         or         yield
assert     else       import     pass
break      except     in         raise

Identifier names

When we write a Python program, we will create many entities – variables which store values like numbers or strings, as well as functions and classes. These entities must given names by which they can be referred to uniquely – these names are known as identifiers. For example, in our skeleton code above, my_function is the name of the function. This particular name has no special significance – we could also have called the function main or print_hello_world. What is important is that we use the same name to refer to the function when we call it at the bottom of the program.
Python has some rules that you must follow when forming an identifier:
  • it may only contain letters (uppercase or lowercase), numbers or the underscore character (_) (no spaces!).
  • it may not start with a number.
  • it may not be a keyword.
If we break any of these rules, our program will exit with a syntax error. However, not all identifiers which are syntactically correct are meaningful to human readers. There are a few guidelines that we should follow when naming our variables to make our code easier to understand (by other people, and by us!) – this is an important part of following a good coding style:
  • be descriptive – a variable name should describe the contents of the variable; a function name should indicate what the function does; etc..
  • don’t use abbreviations unnecessarily – they may be ambiguous and more difficult to read.
Pick a naming convention, and stick to it. This is a commonly used naming convention in Python:
  • names of classes should be in CamelCase (words capitalised and squashed together).
  • names of variables which are intended to be constants should be in CAPITAL_LETTERS_WITH_UNDERSCORES.
  • names of all other variables should be in lowercase_with_underscores. In some other languages, like Java, the standard is to use camelCase (with the initial letter lowercase), but this style is less popular in Python.
  • names of class attributes and methods which are intended to be “private” and not accessed from outside the class should start with an underscore.
Of course there are always exceptions – for example, many common mathematical symbols have very short names which are nonetheless widely understood.
Here are a few examples of identifiers:
Syntax error Bad practice Good practice
Person Record PRcrd PersonRecord
DEFAULT-HEIGHT Default_Ht DEFAULT_HEIGHT
class Class AlgebraCourse
2totalweight num2 total_weight
Note
Be careful not to redefine existing variables accidentally by reusing their names. This applies not only to your own variables, but to built-in Python functions like len, max or sort: these names are not keywords, and you will not get a syntax error if you reuse them, but you will encounter confusing results if you try to use the original functions later in your program. Redefining variables (accidentally and on purpose) will be discussed in greater detail in the section about scope.

Exercise 1

Write down why each of the entries in the left column will raise a syntax error if it is used as an identifier.

Flow of control

In Python, statements are written as a list, in the way that a person would write a list of things to do. The computer starts off by following the first instruction, then the next, in the order that they appear in the program. It only stops executing the program after the last instruction is completed. We refer to the order in which the computer executes instructions as the flow of control. When the computer is executing a particular instruction, we can say that control is at that instruction.

Indentation and (lack of) semicolons

Many languages arrange code into blocks using curly braces ({ and }) or BEGIN and END statements – these languages encourage us to indent blocks to make code easier to read, but indentation is not compulsory. Python uses indentation only to delimit blocks, so we must indent our code:
# this function definition starts a new block
def add_numbers(a, b):
    # this instruction is inside the block, because it's indented
    c = a + b
    # so is this one
    return c

# this if statement starts a new block
if it_is_tuesday:
    # this is inside the block
    print("It's Tuesday!")
# this is outside the block!
print("Print this no matter what.")
In many languages we need to use a special character to mark the end of each instruction – usually a semicolon. Python uses ends of lines to determine where instructions end (except in some special cases when the last symbol on the line lets Python know that the instruction will span multiple lines). We may optionally use semicolons – this is something we might want to do if we want to put more than one instruction on a line (but that is usually bad style):
# These all individual instructions -- no semicolons required!
print("Hello!")
print("Here's a new instruction")
a = 2

# This instruction spans more than one line
b = [1, 2, 3,
    4, 5, 6]

# This is legal, but we shouldn't do it
c = 1; d = 5

Exercise 2

Write down the two statements inside the block created by the append_chickens function:
no_chickens = "No chickens here ..."

def append_chickens(text):
    text = text + " Rawwwk!"
    return text

print(append_chickens(no_chickens))

Exercise 3

The following Python program is not indented correctly. Re-write it so that it is correctly indented:
def happy_day(day):
if day == "monday":
return ":("
if day != "monday":
return ":D"

print(happy_day("sunday"))
print(happy_day("monday"))

Letter case

Unlike some languages (such as Pascal), Python is case-sensitive. This means that the interpreter treats upper- and lowercase letters as different from one another. For example, A is different from a and def main() is different from DEF MAIN(). Also remember that all reserved words (except True, False and None) are in lowercase.

More on Comments

Recall that comments start with # and continue until the end of the line, for example:
# This is a comment
print("Hello!")    # tells the computer to print "Hello!"
Comments are ignored by the interpreter and should be used by a programmer to:
  • describe what the program does
  • describe (in higher-level terms than the code) how the program works
It is not necessary to comment each line. We should comment in appropriate places where it might not be clear what is going on. We can also put a short comment describing what is taking place in the next few instructions following the comment.
Some languages also have support for comments that span multiple lines, but Python does not. If we want to type a very long comment in Python, we need to split it into multiple shorter lines and put a # at the start of each line.
Note
It is possible to insert a multi-line string literal into our code by enclosing it in triple quotes. This is not normally used for comments, except in the special case of docstrings: strings which are inserted at the top of structures like functions and classes, and which document them according to a standard format. It is good practice to annotate our code in this way because automated tools can then parse it to generate documentation automatically. We will discuss docstrings further in a future chapter.
Note
You can easily disable part of your program temporarily by commenting out some lines. Adding or removing many hashes by hand can be time-consuming – your editor should have a keyboard shortcut which allows you to comment or uncomment all the text you have selected.

Reading and writing

Many programs display text on the screen either to give some information or to ask for some information. For example, we might just want to tell the user what our program does:
Welcome to John's Calculating Machine.
Perhaps we might want to ask the user for a number:
Enter the first number:
The easiest way to output information is to display a string literal using the built-in print function. A string literal is text enclosed in quotes. We can use either single quotes (') or double quotes (") – but the start quote and the end quote have to match!
These are examples of string literals:
"Welcome to John's Calculating Machine."
'Enter the first number:'
We can tell the computer to print “Hello!” on the console with the following instruction:
print("Hello!")
As you can see the print function takes in a string as an argument. It prints the string, and also prints a newline character at the end – this is why the console’s cursor appears on a new line after we have printed something.
To query the user for information, we use the input function:
first_number = input('Enter the first number: ')
There are several things to note. First, unlike the print function, the input function does not print a newline automatically – the text will be entered directly after the prompt. That is why we have added a trailing space after the colon. Second, the function always returns a string – we will have to convert it to a number ourselves.
The string prompt is optional – we could just use the input function without a parameter:
second_number = input()
Note
in Python 2, there is a function called raw_input which does what input does in Python 3: that is, it reads input from the user, and returns it as a string. In Python 2, the function called input does something different: it reads input from the user and tries to evaluate it as a Python expression. There is no function like this in Python 3, but you can achieve the same result by using the eval function on the string returned by input. eval is almost always a bad idea, and you should avoid using it – especially on arbitrary user input that you haven’t checked first. It can be very dangerous – the user could enter absolutely anything, including malicious code!

Files

Although the print function prints to the console by default, we can also use it to write to a file. Here is a simple example:
with open('myfile.txt', 'w') as myfile:
    print("Hello!", file=myfile)
Quite a lot is happening in these two lines. In the with statement (which we will look at in more detail in the chapter on errors and exceptions) the file myfile.txt is opened for writing and assigned to the variable myfile. Inside the with block, Hello! followed by a newline is written to the file. The w character passed to open indicates that the file should be opened for writing.
As an alternative to print, we can use a file’s write method as follows:
with open('myfile.txt', 'w') as myfile:
    myfile.write("Hello!")
A method is a function attached to an object – methods will be explained in more detail in the chapter about classes.
Unlike print, the write method does not add a newline to the string which is written.
We can read data from a file by opening it for reading and using the file’s read method:
with open('myfile.txt', 'r') as myfile:
    data = myfile.read()
This reads the contents of the file into the variable data. Note that this time we have passed r to the open function. This indicates that the file should be opened for reading.
Note
Python will raise an error if you attempt to open a file that has not been created yet for reading. Opening a file for writing will create the file if it does not exist yet.
Note
The with statement automatically closes the file at the end of the block, even if an error occurs inside the block. In older versions of Python files had to be closed explicitly – this is no longer recommended. You should always use the with statement.

Built-in types

There are many kinds of information that a computer can process, like numbers and characters. In Python (and other programming languages), the kinds of information the language is able to handle are known as types. Many common types are built into Python – for example integers, floating-point numbers and strings. Users can also define their own types using classes.
In many languages a distinction is made between built-in types (which are often called “primitive types” for this reason) and classes, but in Python they are indistinguishable. Everything in Python is an object (i.e. an instance of some class) – that even includes lists and functions.
A type consists of two parts: a domain of possible values and a set of possible operations that can be performed on these values. For example, the domain of the integer type (int) contains all integers, while common integer operations are addition, subtraction, multiplication and division.
Python is a dynamically (and not statically) typed language. That means that we don’t have to specify a type for a variable when we create it – we can use the same variable to store values of different types. However, Python is also strongly (and not weakly) typed – at any given time, a variable has a definite type. If we try to perform operations on variables which have incompatible types (for example, if we try to add a number to a string), Python will exit with a type error instead of trying to guess what we mean.
The function type can be used to determine the type of an object. For example:
print(type(1))
print(type("a"))

Integers

An integer (int type) is a whole number such as 1, 5, 1350 or -34. 1.5 is not an integer because it has a decimal point. Numbers with decimal points are floating-point numbers. Even 1.0 is a floating-point number and not an integer.

Integer operations

Python can display an integer with the print function, but only if it is the only argument:
print(3)
# We can add two numbers together
print(1 + 2)
We can’t combine a string and an integer directly, because Python is strongly typed:
>>> print("My number is " + 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly
If we want to print a number and a string together, we will have to convert the number to a string somehow:
# str function converts things to strings.
# Then we can concatenate two strings with +.
print("My number is " + str(3))

# String formatting does the conversion for us.
print("My number is %d" % 3)
Other integer operations:
Operation Symbol Example Result
Addition + 28 + 10 38
Subtraction - 28 - 10 18
Multiplication * 28 * 10 280
Division // 28 // 10 2
Modulus (remainder) % 28 % 10 8
Exponent (power) ** 28**10 296196766695424
Note that all these operations are integer operations. That is why the answer to 28 // 10 is not 2.8, but 2. An integer operation results in an integer solution.
Note
In Python 2, the operator / performed integer division if both the dividend and the divisor were integers, and floating-point division if at least one of them was a float. In Python 3, / always performs floating-point division and // always performs integer division – even if the dividend and divisor are floats!
Note
Some other languages (e.g. C, Java) store each integer in a small fixed amount of memory. This limits the size of the integer that may be stored. Common limits are 2**8, 2**16, 2**32 and 2**64. Python has no fixed limit can stored surprisingly large integers such as 2**1000000 as long as there is enough memory and processing power available on the machine where it is running.

Operator precedence

Another important thing to keep in mind is operator precedence. For example, does 1 + 2 // 3 mean (1 + 2) // 3 or 1 + (2 // 3)? Python has a specific and predictable way to determine the order in which it performs operations. For integer operations, the system will first handle brackets (), then **, then *, // and %, and finally + and -.
If an expression contains multiple operations which are at the same level of precedence, like *, // and %, they will be performed in order, either from left to right (for left-associative operators) or from right to left (for right-associative operators). All these arithmetic operators are left-associative, except for **, which is right-associative:
# all arithmetic operators other than ** are left-associative, so
2 * 3 / 4
# is evaluated left to right:
(2 * 3) / 4

# ** is right-associative, so
2 ** 3 ** 4
# is evaluated right to left:
2 ** (3 ** 4)
The following table shows some more examples of precedence:
Expression How Python evaluates Result
20 + 10 // 2 20 + (10 // 2) 25
20 + 10 - 2 (20 + 10) - 2 28
20 - 10 + 2 (20 - 10) + 2 12
20 - 10 * 2 20 - (10 * 2) 0
20 // 10 * 2 (20 // 10) * 2 4
20 * 10 // 2 (20 * 10) // 2 100
20 * 10 ** 2 20 * (10 ** 2) 2000
Sometimes it’s a good idea to add brackets to arithmetic expressions even if they’re not compulsory, because it makes the code more understandable.

Exercise 4

  1. Which of the following numbers are valid Python integers? 110, 1.0, 17.5, -39, -2.3
  2. What are the results of the following operations and explain why: #. 15 + 20 * 3 #. 13 // 2 + 3 #. 31 + 10 // 3 #. 20 % 7 // 3 #. 2 ** 3 ** 2
  3. What happens when you evaluate 1 // 0 in the Python console? Why does this happen?

Floating-point numbers

Floating-point numbers (float type) are numbers with a decimal point or an exponent (or both). Examples are 5.0, 10.24, 0.0, 12. and .3. We can use scientific notation to denote very large or very small floating-point numbers, e.g. 3.8 x 1015. The first part of the number, 3.8, is the mantissa and 15 is the exponent. We can think of the exponent as the number of times we have to move the decimal point to the right to get to the actual value of the number.
In Python, we can write the number 3.8 x 1015 as 3.8e15 or 3.8e+15. We can also write it as 38e14 or .038e17. They are all the same value. A negative exponent indicates smaller numbers, e.g. 2.5e-3 is the same as 0.0025. Negative exponents can be thought of as how many times we have to move the decimal point to the left. Negative mantissa indicates that the number itself is negative, e.g. -2.5e3 equals -2500 and -2.5e-3 equals -0.0025.
The print function will display floating-point numbers in decimal notation if they are greater than or equal to 1e-4 and less than 1e16, but for smaller and larger numbers it will use scientific notation:
# This will print 10000000000.0
print(1e10)

# This will print 1e+100
print(1e100)

# This will print 1e-10
print(0.0000000001)
When displaying floats, we will usually specify how we would like them to be displayed, using string formatting:
# This will print 12.35
print("%.2f" % 12.3456)

# This will print 1.234560e+01
print("%e" % 12.3456)
Note that any rounding only affects the display of the numbers. The precision of the number itself is not affected.

Floating-point operations and precedence

Arithmetic operations for floating-point numbers are the same as those for integers: addition, subtraction, multiplication, division and modulus. They also use the same operators, except for division – the floating-point division operator is /. Floating-point operations always produce a floating-point solution. The order of precedence for these operators is the same as those for integer operators.
Often, we will have to decide which type of number to use in a program. Generally, we should use integers for counting and measuring discrete whole numbers. We should use floating-point numbers for measuring things that are continuous.
We can combine integers and floating-point numbers in arithmetic expressions without having to convert them – this is something that Python will do for us automatically. If we perform an arithmetic operation on an integer and a floating-point number, the result will always be a floating-point number.
We can use the integer division operator on floating-point numbers, and vice versa. The two division operators are at the same level in the order of precedence.
Note
Python floating-point numbers conform to a standardised format named IEEE 754. The standard represents each floating-point number using a small fixed amount of memory, so unlike Python’s integers, Python’s floating-point numbers have a limited range. The largest floating-point number that can be represented in Python is 2**1023.
Note
Python includes three other types for dealing with numbers:
  • complex (like floating point but for complex numbers; try 1+5j)
  • Fraction (for rational numbers; available in the fractions module)
  • Decimal (for decimal floating-point arithmetic; available in the decimal module).
Using these is beyond the scope of this module, but it’s worth knowing that they exist in case you have a use for them later.

Exercise 5

  1. Which of the following are Python floating-point numbers? 1, 1.0, 1.12e4, -3.141759, 735, 0.57721566, 7.5e-3
  2. What is the difference between integer and floating-point division? What is the operator used for integer division? What is the operator used for floating-point division?
  3. What are the results of the following operations? Explain why: #. 1.5 + 2 #. 1.5 // 2.0 #. 1.5 / 2.0 #. 1.5 ** 2 #. 1 / 2 #. -3 // 2
  4. What happens when you evaluate 1 / 0 in the Python console?
  5. What happens when you evaluate 1e1000? What about -1e1000? And type(1e1000)?

Strings

A string is a sequence of characters. You should already be familiar with string literals from working with them in the last section. In Python, strings (type str) are a special kind of type which is similar to sequence types. In many ways, strings behave in similar ways to lists (type list), which we will discuss in a later chapter, but they also have some functionality specific to text.
Many other languages have a different variable type for individual characters – but in Python single characters are just strings with a length of 1.
Note
In Python 2, the str type used the ASCII encoding. If we wanted to use strings containing Unicode (for example, characters from other alphabets or special punctuation) we had to use the unicode type. In Python 3, the str type uses Unicode.

String formatting

We will often need to print a message which is not a fixed string – perhaps we want to include some numbers or other values which are stored in variables. The recommended way to include these variables in our message is to use string formatting syntax:
name = "Jane"
age = 23
print("Hello! My name is %s." % name)
print("Hello! My name is %s and I am %d years old." % (name, age))
The symbols in the string which start with percent signs (%) are placeholders, and the variables which are to be inserted into those positions are given after the string formatting operator, %, in the same order in which they appear in the string. If there is only one variable, it doesn’t require any kind of wrapper, but if we have more than one we need to put them in a tuple (between round brackets). The placeholder symbols have different letters depending on the type of the variable – name is a string, but age is an integer. All the variables will be converted to strings before being combined with the rest of the message.

Escape sequences

An escape sequence (of characters) can be used to denote a special character which cannot be typed easily on a keyboard or one which has been reserved for other purposes. For example, we may want to insert a newline into our string:
print('This is one line.\nThis is another line.')
If our string is enclosed in single quotes, we will have to escape apostrophes, and we need to do the same for double quotes in a string enclosed in double quotes. An escape sequence starts with a backslash (\):
print('"Hi! I\'m Jane," she said.')
print("\"Hi! I'm Jane,\" she said.")
If we did not escape one of these quotes, Python would treat it as the end quote of our string – and shortly afterwards it would fail to parse the rest of the statement and give us a syntax error:
>>> print('"Hi! I'm Jane," she said.')
  File "<stdin>", line 1
    print('"Hi! I'm Jane," she said.')
                  ^
SyntaxError: invalid syntax
Some common escape sequences:
Sequence Meaning
\\ literal backslash
\' single quote
\" double quote
\n newline
\t tab
We can also use escape sequences to output unicode characters.

Raw strings

Sometimes we may need to define string literals which contain many backslashes – escaping all of them can be tedious. We can avoid this by using Python’s raw string notation. By adding an r before the opening quote of the string, we indicate that the contents of the string are exactly what we have written, and that backslashes have no special meaning. For example:
# This string ends in a newline
"Hello!\n"

# This string ends in a backslash followed by an 'n'
r"Hello!\n"
We most often use raw strings when we are passing strings to some other program which does its own processing of special sequences. We want to leave all such sequences untouched in Python, to allow the other program to handle them.

Triple quotes

In cases where we need to define a long literal spanning multiple lines, or containing many quotes, it may be simplest and most legible to enclose it in triple quotes (either single or double quotes, but of course they must match). Inside the triple quotes, all whitespace is treated literally – if we type a newline it will be reflected in our string. We also don’t have to escape any quotes. We must be careful not to include anything that we don’t mean to – any indentation will also go inside our string!
These string literals will be identical:
string_one = '''"Hello," said Jane.
"Hi," said Bob.'''

string_two = '"Hello," said Jane.\n"Hi," said Bob.'

String operations

We have already introduced a string operation - concatenation (+). It can be used to join two strings. There are many built-in functions which perform operations on strings. String objects also have many useful methods (i.e. functions which are attached to the objects, and accessed with the attribute reference operator, .):
name = "Jane Smith"

# Find the length of a string with the built-in len function
print(len(name))

# Print the string converted to lowercase
print(name.lower())
# Print the original string
print(name)
Why does the last print statement output the original value of name? It’s because the lower method does not change the value of name. It returns a modified copy of the value. If we wanted to change the value of name permanently, we would have to assign the new value to the variable, like this:
# Convert the string to lowercase
name = name.lower()
print(name)
In Python, strings are immutable – that means that we can’t modify a string once it has been created. However, we can assign a new string value to an existing variable name.

Exercise 6

  1. Given variables x and y, use string formatting to print out the values of x and y and their sum. For example, if x = 5 and y = 3 your statement should print 5 + 3 = 8.
  2. Re-write the following strings using single-quotes instead of double-quotes. Make use of escape sequences as needed: #. "Hi! I'm Eli." #. "The title of the book was \"Good Omens\"." #. "Hi! I\'m Sebastien."
  3. Use escape sequences to write a string which represents the letters a, b and c separated by tabs.
  4. Use escape sequences to write a string containing the following haiku (with newlines) inside single double-or-single quotes. Then do the same using triple quotes instead of the escape sequences:
    the first cold shower
even the monkey seems to want
a little coat of straw
  5. Given a variable name containing a string, write a print statement that prints the name and the number of characters in it. For example, if name = "John", your statement should print John's name has 4 letters..
  6. What does the following sequence of statements output:
    name = "John Smythe"
print(name.lower())
print(name)
    Why is the second line output not lowercase?

Answers to exercises

Answer to exercise 1

Syntax error Reason
Person Record Identifier contains a space.
DEFAULT-HEIGHT Identifier contains a dash.
class Identifier is a keyword.
2totalweight Identifier starts with a number.

Answer to exercise 2

The two statements inside the block defined by the append_chickens function are:
text = text + " Rawwwk!"
return text

Answer to exercise 3

The correctly indented code is:
def happy_day(day):
    if day == "monday":
        return ":("
    if day != "monday":
        return ":D"

print(happy_day("sunday"))
print(happy_day("monday"))

Answer to exercise 4

  1. The valid Python integers are: 110 and -39
    1. 15 + 20 * 3: 75 – * has higher precedence than +.
    2. 13 // 2 + 3: 9 – // has higher precedence than +.
    3. 31 + 10 // 3: 34 – as above.
    4. 20 % 7 // 3: 2 – // and % have equal precedence but are left-associative (so the left-most operation is performed first).
    5. 2 ** 3 ** 2: 512 – ** is right-associative so the right-most exponential is performed first.
  3. A ZeroDivisionError is raised.

Answer to exercise 5

#. Only 1 and 735 are not floating-point numbers (they are integers).
  1. In integer division the fractional part (remainder) is discarded (although the result is always a float if one of the operands was a float). The Python operator for integer division is //. The operator for floating-point division is /.
    1. 1.5 + 2: 3.5 – the integer 2 is converted to a floating-point number and then added to 1.5.
    2. 1.5 // 2.0: 0.0 – integer division is performed on the two floating-point numbers and the result is returned (also as a floating-point number).
    3. 1.5 / 2.0: 0.75 – floating-point division is performed on the two numbers.
    4. 1.5 ** 2: 2.25
    5. 1 / 2: 0.5 – floating-point division of two integers returns a floating-point number.
    6. -3 // 2: -2 – integer division rounds the result down even for negative numbers.
  3. A ZeroDivisionError is raised. Note that the error message is slightly different to the one returned by 1 // 0.
  4. 1e1000 is too large to be represented as a floating-point number. Instead the special floating-point value inf is returned (inf is short for infinity). As you will have noticed by inspecting its type, inf is really a floating-point number in Python (and not the string "inf"). -1e1000 gives a different special floating-point value, -inf, which is short for minus infinity). These special values are defined by the IEEE 754 floating-point specification that Python follows.

Answer to exercise 6

  1. One possible print statement is:
    print("%s + %s = %s" % (x, y, x + y))
  2. The equivalent single-quoted strings are: #. 'Hi! I\'m Eli.' #. 'The title of the book was "Good Omens".' #. 'Hi! I\'m Sebastien.'
  3. "a\tb\tc"
  4. Using single double-quotes:
    "the first cold shower\neven the monkey seems to want\na little
coat of straw"
    Using triple quotes:
    """the first cold shower
even the monkey seems to want
a little coat of straw"""
  5. print("%s's name has %s letters." % (name, len(name)))
  6. The output is:
    john smythe
John Smythe
    The second line is not lowercase because Python strings are immutable and name.lower() returns a new string containing the lowercased name.