# Python

Python is a general purpose programming language that can be used effectively to build any kind of program. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. Python’s elegant syntax and dynamic typing, together with its interpreted nature, make it an ideal language for scripting and rapid application development in many areas on most platforms.Click to download python.

# Literals

Python works by figuring out the meaning or value of some code. This is true for the tiniest pieces of code to the largest programs. The process of finding out the meaning of code is known as

*evaluation*.
The things whose values are the things themselves are known as

*literals*. The literals of Python can be categorized by the following types:*integers*,*real**numbers*,*strings*,**Booleans**, and*arrays*.
Python (or more correctly, the Python interpreter) responds to literals by echoing back the literal itself. Here are examples of each of the types:

```
>>> 3
3
>>> -4.9
-4.9
>>> "hello"
'hello'
```

```
>>>[3, -4.9, "hello"]
[3, -4.9, 'hello']
```

## Integers

Integers are numbers without any fractional parts. Examples of integers are:

```
>>> 3
3
```

## Real Numbers

Reals are numbers that do have a fractional part (even if that fractional part is zero!). Examples of real numbers are:`>>> 3.2 3.2`

## Strings

Strings are sequences of characters delineated by double quotation marks:`>>> "hello, world!" 'hello, world!'`

`Python accepts both double quotes and single quotes to delineate strings.Characters in a string can be`

escaped(or quoted) with the backslash character, which changes the meaning of some characters. For example, the charactern, in a string refers to the letternwhile the character sequence`\`

nrefers to thenewlinecharacter. A backslash also changes the meaning of the lettert, converting it into a tab character. You can also quote single and double quotes with backslashes. When other characters are escaped, it is assumed the backslash is a character of the string and it is escaped (with a backslash) in the result:`>>> "\z" '\\z'`

A string with no characters between the double quotes is known as an empty string.Unlike some languages, there is no character type in Python. A single charactera, for example, is entered as the string`"a"`

or`'a'`

.## True, False, and None

There are two special literals,`True`

and`False`

. These literals are known as theBooleanvalues and are used to guide the flow of a program. The so-calledBooleanlogic orBooleanalgebra is concerned with the rules of combining truth values (i.e., true or false).Another special literal is`None`

. This literal is used to indicate the end of lists; it also is used to indicate something that has not yet been created.## Collections of literals

If you read any other text on Python, the basic way of grouping literals together (rather like throwing a bunch of groceries in a shopping bag) is called alist.Arrays are just collections of values. One creates an array by enclosing a comma-separated listing of values in square brackets. The simplest array is empty:`>>>[] []`

Arrays can contain any values:`>>>[2, "help", len] [2, 'help', <built-in function len>]`

The first value is an integer, the second a string, and the third is something known as a function. We will learn more about functions later, but thelenfunction is used to tell us how many items are in an array:`>>> len([2, "help", len]) 3`

Arrays can even contain arrays!`>>>[0, [3, 2, 1] 4] [0, [3, 2, 1] 4]`

An array is something known as adata structure; data structures are extremely important in writing sophisticated programs.## Indexing into Arrays

You can pull out an item from an array by usingbracket notation. With bracket notation, you specify exactly which element (or elements) you wish to extract from the array. This specification is called anindex. The first element of the array has index 0, the second index 1, and so on. Here is some code that extracts the first element of an array. Note that the first interaction creates avariablenamed items that points to an array of three elements.`>>> items = ['a', True, 7] >>> items ['a', True, 7] >>> items[0] 'a' >>> items[1] True`

Extracting an item from an array leaves the array unchanged. What happens if our index is too large?`>>> items[3] Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range`

There is a special notation for extractingmore than oneelement of an array. This notation is known as aslice. Here is a slice that extracts all but the first element of an array:`>>> items[1:] [True, 7]`

`This particular slice (you can slice an array many different ways) says, start extracting at the second item (which has index one) and go to the end. This operation is so common, it has a special name, taking the`

tailof an array.Here is a slice that says, start extracting at the first element (which has index 0) and go up to, but do not include, the element with index 2:`>>> items[0:2] ['a', True]`

## Operators

The operands of the other basic operators have special names too. For addition, the left operand is known as theaugendand the right operand is known as theaddend. The result is known as thesum. For subtraction, the left operand is theminuend, the right thesubtrahend, and the result as thedifference. For division (and I think this is still taught), the left operand is thedividend, the right operand is thedivisor, and the result is thequotient. Finally, for exponentiation, which is shorthand for repeated multiplication:`>>> 3 ** 4 81 >>> 3 * 3 * 3 * 3 81`

the left operand is thebaseand the right operand is theexponentIf it makes sense to add two things together, you can probably do it in Python using the + operator. For example:`>>> 2 + 3 5`

Adding an string to an integer (with an augend integer) yields an error; the types are not "close" enough, like they are with integers and reals:`>>> 2 + "hello" TypeError: unsupported operand types(s) for +: 'int' and 'str'`

In general, when adding two things, the types must match or nearly match.>>> 1.9 + 3.1 5.0One can see that if one adds two integers, the result is an integer.You can even add strings with strings and arrays with arrays:`>>> "hello" + "world" 'helloworld' >>> [1, 3, 5] + [2, 4, 6] [1, 3, 5, 2, 4, 6]`

You can multiply strings and arrays with numbers:`>>> "hello" * 3 'hellohellohello' >>> [1, 2] * 3 [1, 2, 1, 2, 1, 2]`

The division operator with respect to integer operands. Consider evaluating the following expression:`15 / 2`

If one asked the Python interpreter to perform this task, the result would be 7.5, as expected. However, often we wish for just the quotient without the remainder. In this case, the quotient is 7 and the remainder is 0.5. The double forward slash operator is Python's quotient operator; if we ask the interpreter to evaluate`14 // 5`

the result would be 2, not 2.8. Use of the quotient operator is known asintegerdivision.The complement to integer division is the modulus operator %. While the result of integer division is the quotient, the result of the modulus operator is the remainder. Thus`14 % 5`

evaluates to 4 since 4 is left over when 5 is divided into 14. To check if this is true, one can ask the interpreter to evaluate:`(14 // 5 * 5) + (14 % 5) == 14`

`This complicated expression asks the question "is it true that the quotient times the divisor plus the remainder is equal to the original dividend?". The Python interpreter will respond that, indeed, it is true.`

## Comparing things

TheBooleanliterals,TrueandFalsecan be used in theBooleancomparison operators to generate such values. For example, we can ask if 3 is less than 4:`>>> 3 < 4 True`

Besides integers, we can compare reals with reals, strings with strings, and arrays with arrays using the comparison operators:`>>> "apple" < "banana" True >>> [1, 2, 3] < [1, 2, 4] True`

`In general, it is illegal to compare integers or reals with strings.`

`>>> not(3 < 4 and 4 < 5) False >>> not(3 < 4 or 4 < 5) False`

## Precedence:

+and-have higher precedence than<. The lowest precedence operator in Python is the assignment operator Next come the Boolean connectivesandandor. At the next higher level are the Boolean comparatives,<,<=,>,>=,==, and!=.After that come the additive arithmetic operators+and-. Next comes the multiplicative operators*,/and%. Higher still is the exponentiation operator**. Finally, at the highest level of precedence is the selection, ordot, operator (the dot operator is a period or full-stop). Higher precedence operations are performed before lower precedence operations. Functions which are called with operator syntax have the same precedence level as the mathematical operators.## Assignment and Arrays

You can change a particular element of a array by assigning a new value to the index of that element by using bracket notation:`>>> items = ['a', True, 7] >>> items[0] = 'b'`

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