7. Input and Output¶
There are several ways to present the output of a program; data can be printed in a human-readable form, or written to a file for future use. This chapter will discuss some of the possibilities.
7.1. Fancier Output Formatting¶
So far we’ve encountered two ways of writing values: expression statements and the print() function. (A third way is using the write() method of file objects; the standard output file can be referenced as sys.stdout . See the Library Reference for more information on this.)
Often you’ll want more control over the formatting of your output than simply printing space-separated values. There are several ways to format output.
To use formatted string literals , begin a string with f or F before the opening quotation mark or triple quotation mark. Inside this string, you can write a Python expression between < and >characters that can refer to variables or literal values.
The str.format() method of strings requires more manual effort. You’ll still use < and >to mark where a variable will be substituted and can provide detailed formatting directives, but you’ll also need to provide the information to be formatted.
Finally, you can do all the string handling yourself by using string slicing and concatenation operations to create any layout you can imagine. The string type has some methods that perform useful operations for padding strings to a given column width.
When you don’t need fancy output but just want a quick display of some variables for debugging purposes, you can convert any value to a string with the repr() or str() functions.
The str() function is meant to return representations of values which are fairly human-readable, while repr() is meant to generate representations which can be read by the interpreter (or will force a SyntaxError if there is no equivalent syntax). For objects which don’t have a particular representation for human consumption, str() will return the same value as repr() . Many values, such as numbers or structures like lists and dictionaries, have the same representation using either function. Strings, in particular, have two distinct representations.
The string module contains a Template class that offers yet another way to substitute values into strings, using placeholders like $x and replacing them with values from a dictionary, but offers much less control of the formatting.
7.1.1. Formatted String Literals¶
Formatted string literals (also called f-strings for short) let you include the value of Python expressions inside a string by prefixing the string with f or F and writing expressions as
An optional format specifier can follow the expression. This allows greater control over how the value is formatted. The following example rounds pi to three places after the decimal:
Passing an integer after the ‘:’ will cause that field to be a minimum number of characters wide. This is useful for making columns line up.
Other modifiers can be used to convert the value before it is formatted. ‘!a’ applies ascii() , ‘!s’ applies str() , and ‘!r’ applies repr() :
The = specifier can be used to expand an expression to the text of the expression, an equal sign, then the representation of the evaluated expression:
See self-documenting expressions for more information on the = specifier. For a reference on these format specifications, see the reference guide for the Format Specification Mini-Language .
7.1.2. The String format() Method¶
Basic usage of the str.format() method looks like this:
The brackets and characters within them (called format fields) are replaced with the objects passed into the str.format() method. A number in the brackets can be used to refer to the position of the object passed into the str.format() method.
If keyword arguments are used in the str.format() method, their values are referred to by using the name of the argument.
Positional and keyword arguments can be arbitrarily combined:
If you have a really long format string that you don’t want to split up, it would be nice if you could reference the variables to be formatted by name instead of by position. This can be done by simply passing the dict and using square brackets ‘[]’ to access the keys.
This could also be done by passing the table dictionary as keyword arguments with the ** notation.
This is particularly useful in combination with the built-in function vars() , which returns a dictionary containing all local variables.
As an example, the following lines produce a tidily aligned set of columns giving integers and their squares and cubes:
For a complete overview of string formatting with str.format() , see Format String Syntax .
7.1.3. Manual String Formatting¶
Here’s the same table of squares and cubes, formatted manually:
(Note that the one space between each column was added by the way print() works: it always adds spaces between its arguments.)
The str.rjust() method of string objects right-justifies a string in a field of a given width by padding it with spaces on the left. There are similar methods str.ljust() and str.center() . These methods do not write anything, they just return a new string. If the input string is too long, they don’t truncate it, but return it unchanged; this will mess up your column lay-out but that’s usually better than the alternative, which would be lying about a value. (If you really want truncation you can always add a slice operation, as in x.ljust(n)[:n] .)
There is another method, str.zfill() , which pads a numeric string on the left with zeros. It understands about plus and minus signs:
7.1.4. Old string formatting¶
The % operator (modulo) can also be used for string formatting. Given ‘string’ % values , instances of % in string are replaced with zero or more elements of values . This operation is commonly known as string interpolation. For example:
More information can be found in the printf-style String Formatting section.
7.2. Reading and Writing Files¶
open() returns a file object , and is most commonly used with two positional arguments and one keyword argument: open(filename, mode, encoding=None)
The first argument is a string containing the filename. The second argument is another string containing a few characters describing the way in which the file will be used. mode can be ‘r’ when the file will only be read, ‘w’ for only writing (an existing file with the same name will be erased), and ‘a’ opens the file for appending; any data written to the file is automatically added to the end. ‘r+’ opens the file for both reading and writing. The mode argument is optional; ‘r’ will be assumed if it’s omitted.
Normally, files are opened in text mode, that means, you read and write strings from and to the file, which are encoded in a specific encoding. If encoding is not specified, the default is platform dependent (see open() ). Because UTF-8 is the modern de-facto standard, encoding="utf-8" is recommended unless you know that you need to use a different encoding. Appending a ‘b’ to the mode opens the file in binary mode. Binary mode data is read and written as bytes objects. You can not specify encoding when opening file in binary mode.
In text mode, the default when reading is to convert platform-specific line endings ( \n on Unix, \r\n on Windows) to just \n . When writing in text mode, the default is to convert occurrences of \n back to platform-specific line endings. This behind-the-scenes modification to file data is fine for text files, but will corrupt binary data like that in JPEG or EXE files. Be very careful to use binary mode when reading and writing such files.
It is good practice to use the with keyword when dealing with file objects. The advantage is that the file is properly closed after its suite finishes, even if an exception is raised at some point. Using with is also much shorter than writing equivalent try — finally blocks:
If you’re not using the with keyword, then you should call f.close() to close the file and immediately free up any system resources used by it.
Calling f.write() without using the with keyword or calling f.close() might result in the arguments of f.write() not being completely written to the disk, even if the program exits successfully.
After a file object is closed, either by a with statement or by calling f.close() , attempts to use the file object will automatically fail.
7.2.1. Methods of File Objects¶
The rest of the examples in this section will assume that a file object called f has already been created.
To read a file’s contents, call f.read(size) , which reads some quantity of data and returns it as a string (in text mode) or bytes object (in binary mode). size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned; it’s your problem if the file is twice as large as your machine’s memory. Otherwise, at most size characters (in text mode) or size bytes (in binary mode) are read and returned. If the end of the file has been reached, f.read() will return an empty string ( » ).
f.readline() reads a single line from the file; a newline character ( \n ) is left at the end of the string, and is only omitted on the last line of the file if the file doesn’t end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by ‘\n’ , a string containing only a single newline.
For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:
If you want to read all the lines of a file in a list you can also use list(f) or f.readlines() .
f.write(string) writes the contents of string to the file, returning the number of characters written.
Other types of objects need to be converted – either to a string (in text mode) or a bytes object (in binary mode) – before writing them:
f.tell() returns an integer giving the file object’s current position in the file represented as number of bytes from the beginning of the file when in binary mode and an opaque number when in text mode.
To change the file object’s position, use f.seek(offset, whence) . The position is computed from adding offset to a reference point; the reference point is selected by the whence argument. A whence value of 0 measures from the beginning of the file, 1 uses the current file position, and 2 uses the end of the file as the reference point. whence can be omitted and defaults to 0, using the beginning of the file as the reference point.
In text files (those opened without a b in the mode string), only seeks relative to the beginning of the file are allowed (the exception being seeking to the very file end with seek(0, 2) ) and the only valid offset values are those returned from the f.tell() , or zero. Any other offset value produces undefined behaviour.
File objects have some additional methods, such as isatty() and truncate() which are less frequently used; consult the Library Reference for a complete guide to file objects.
7.2.2. Saving structured data with json ¶
Strings can easily be written to and read from a file. Numbers take a bit more effort, since the read() method only returns strings, which will have to be passed to a function like int() , which takes a string like ‘123’ and returns its numeric value 123. When you want to save more complex data types like nested lists and dictionaries, parsing and serializing by hand becomes complicated.
Rather than having users constantly writing and debugging code to save complicated data types to files, Python allows you to use the popular data interchange format called JSON (JavaScript Object Notation). The standard module called json can take Python data hierarchies, and convert them to string representations; this process is called serializing. Reconstructing the data from the string representation is called deserializing. Between serializing and deserializing, the string representing the object may have been stored in a file or data, or sent over a network connection to some distant machine.
The JSON format is commonly used by modern applications to allow for data exchange. Many programmers are already familiar with it, which makes it a good choice for interoperability.
If you have an object x , you can view its JSON string representation with a simple line of code:
Another variant of the dumps() function, called dump() , simply serializes the object to a text file . So if f is a text file object opened for writing, we can do this:
To decode the object again, if f is a binary file or text file object which has been opened for reading:
JSON files must be encoded in UTF-8. Use encoding="utf-8" when opening JSON file as a text file for both of reading and writing.
This simple serialization technique can handle lists and dictionaries, but serializing arbitrary class instances in JSON requires a bit of extra effort. The reference for the json module contains an explanation of this.
pickle — the pickle module
Contrary to JSON , pickle is a protocol which allows the serialization of arbitrarily complex Python objects. As such, it is specific to Python and cannot be used to communicate with applications written in other languages. It is also insecure by default: deserializing pickle data coming from an untrusted source can execute arbitrary code, if the data was crafted by a skilled attacker.
User Input & Data Types in Python
“Python has been an important part of Google since the beginning, and remains so as the system grows and evolves. Today dozens of Google engineers use Python, and we’re looking for more people with skills in this language.” said Peter Norvig, director of search quality at Google, Inc.
“Python is fast enough for our site and allows us to produce maintainable features in record times, with a minimum of developers.” said Cuong Do, Software Architect, YouTube.com.
The implementation of python is widespread and most preferred. In this blog we’ll dive into the most significant components of python programming namely Data types and user input.
Introduction to Python User Input
While programming very often, we are required to take input from the user so as to manipulate the data and process it thereafter. This ensures the program’s dynamism and robustness as it is not hardcoded and can successfully execute at any random value given by the user
Python has two methods of inputting data :
- input()
- raw_input()
A point to note is that, raw_input was used in older versions of Python, and it got replaced by input() in recent Python versions.
Working of Python Input()
When we use the input() function in our program, the flow of execution is halted till the user inputs the data and clicks the Enter button. After that, the user’s value input is stored in some variable in the form of a string. No matter what data type the user intended their data to be, it gets stored as a string only. It needs to explicitly typecast to the desired format before use. Later we will look at such an example where if we do not typecast the data, it will result in an error.
Now let us have a look at the syntax of input()
Syntax
input([<prompt from user>])
Let’s take a quick look at an example for inputting normal text from the keyboard
print("Enter number to find the square:")
number = input()
number = int(number)
print("The square of the number <> is <>".format(number,number**2))
Output
next time you write a Python program, do remember to use the concepts of input() and test your program on many random user input data.
Moving on to Data Types in python
Data types are nothing but the different types of input data accepted by a programming language for defining, declaring, storing and performing mathematical & logical values/ operations. In Python, there are many data types used for dealing with the general operations on the program developer’s input data. A few of the commonly used data types are numbers for numeric values, String for single or series of characters, Tuple for a combination of different data types, List for a collection of values, etc.
Python Data Types
- Numbers
- String
- Tuple
- Set
- Dictionary
- List
The elaboration of the six data types in python are as follows :
Numbers: The Number data type is used to store numeric values.
Consider : var a = 100, var b = 200 # var a and var b number are assigned and these are objects of number. The Number can have 4 types of numeric data:
- int : int stores integers eg a=100, b=25, c=526, etc.
- long: long stores higher range of integers eg a=908090999L, b=-0x1990999L, etc.
- float: float stores floating-point numbers eg a=25.6, b=45.90, c=1.290, etc.
- complex: complex stores numbers eg a=3 + 4j, b=2 + 3j, c=complex(4,6), etc.
String: String data type is used to store the sequence of characters.
The following example shows the string handling with inbuilt operators and functions:
Code:
s = 'hello! how are you' # s is string variable
print (s[1]) # index operator — printing second character, character start storing from index 0
print (s[2:6]) # slice operator — printing 3rd character to 5th character of the string, the syntax of slice operator str[ start: end-1: increment] print (s*3) # printing the string three times
print (s[2:12:2])
s1 = 'hello world'
print (s + s1) # printing the concatenation of s and s1
output :
Tuple: Tuple data type is used to stores a collection of different data types of elements, and it is immutable.
To create a tuple uses the () simple parenthesis; within this brackets, stores all the elements separated with the comma (,).
The following example shows the tuple handling:
Code:
tp = ("apple", "a", 100, 20.78)
print (tp[1])
print (tp[1:])
print (tp[:3])
print (tp)
print (tp + tp)
print (tp * 3)
print (type(tp))
tp[1] = "banana"
print (tp)
output:
List: List data type is used to store the collection of different data types of elements, and it is mutable.
To create a list uses the [] square brackets; within these brackets, stores all the elements separated with the comma (,). We can use index[i], slice [:] operators, concatenation operator (+), repetition operator (*) etc., to works with the list the same as with the strings.
The following example shows the list handling:
Code:
ls = ["apple", "a", 100, 20.78] print (ls[1])
print (ls[1:])
print (ls[:3])
print (ls)
print (ls + ls)
print (ls * 3)
print (type(ls))
ls[1] = "banana"
print (ls)
Output:
Set: Set data type is used to stores the collection of different data types of elements; it is mutable and store unique elements.
To create a set uses the <> curly brackets, within this brackets stores all the elements separated with the comma (,).
The following example shows the set handling:
Code:
st = <"apple", "banana", 100, 20.78>
# set cannot support indexing st[1] # set cannot support slicing st[1:] print (st)
print (st + st)# set cannot support concatenation
print (st * 2) # set cannot support repetition
print (type(st))
# set is immutable st[2] = "hi"
Output:
Dictionary: Dictionary data type is used to store a collection of different data types of elements in the form of key-value pairs; it is mutable and stores the unique key.
Dictionary is also stored in a collection of different data types elements in the form of key-value pairs. It is ordered, mutable and stores unique keys as a set To create a set, uses the <> curly brackets same as a set, within this, brackets stores all the elements (key-value pair) separated with the comma (,).
The following example shows the set handling in Dictionary :
Code:
dc = <"fruits":["apple", "banana"],'qty':100>
print("Fruits: ",dc['fruits'])
print("Quantity: ", dc['qty'])
print ("Dictionary: ",dc)# print all elements of the dictionary
print ("Keys: ",dc.keys()) # print all the keys of the dictionary
print ("values: ",dc.values()) # print all the values of the dictionary
print ("key value pairs: ",dc.items()) # print all the key values pair elements of the dictionary
Output:
Conclusion :
This is a guide to Python User Input and various data types in Python. Here we discuss the methods, working, and the examples along with the appropriate syntax, to receive a complete understanding of these significant concepts.
Ввод, вывод и импорт в Python
В Python существует множество встроенных функций — использовать их можно прямо в командной строке.
Функции вроде input() и print() используются для операций ввода и вывода. Подробное знакомство мы начнем с вывода.
Вывод
Функция print() используется для вывода данных на экран. Эти данные мы можем записать и в файл, но об этом мы поговорим позже.
Пример использования
Вывод:
Еще один пример
Вывод:
Как вы могли заметить, во втором примере с print() между строкой и переменной стоит пробел. По умолчанию вывод происходит именно так, но это можно изменить.
Синтаксис
- objects — то, что мы хотим вывести.
- sep — разделитель между переменными. По умолчанию это пробел.
- end — то, чем кончается строка. По умолчанию это переход на новую строку.
- file — объект, указывающий, куда нужно производить вывод. По умолчанию данные выводятся на экран — sys.stdout .
Примеры
Вывод:
Форматирование вывода
Иногда нужно отформатировать вывод, чтобы он выглядел соответствующим образом. Для этого есть метод str.format() . Его можно использовать с любым строковым объектом.
Фигурные скобки <> здесь выступают в виде заполнителей. Порядок их вывода можно изменять с помощью индексов (индексов кортежа).
Вывод:
Для форматирования строк можно использовать именованные аргументы.
Для форматирования строк можно использовать sprintf() . Это старый способ, который использовался еще в языке Си. Для этого используется оператор % .
До этого момента все наши программы были статическими — все значения переменных мы объявляли заранее.
Теперь нам нужно больше свободы — возможно, мы захотим получить данные от пользователя. В Python для этих целей существует функция input() . Ее синтаксис выглядит так:
promt — это строка, которую мы хотим вывести на экран.
Как можно заметить, введенное значение — строка, а не число. Преобразовать это значение в число можно с помощью функции int() или float() .
Эту же операцию можно выполнить с помощью функции eval() . У eval() есть преимущество — эта функция может проводить расчеты даже если в качестве аргумента передана строка.
Импорт модулей
По мере количества строк вашего кода будет не лишним начинать пользоваться модулями.
Модуль — это файл, содержащий функции и операторы. У всех модулей в Python есть имя, которое заканчивается расширением .py .
Операторы внутри модуля могут быть импортированы в другой модуль или в интерпретатор Python. Для этого мы используем ключевое слово import .
Например, мы можем импортировать модуль math . Делается это следующим образом:
Пример использованияя
Вывод:
Теперь все переменные внутри модуля math доступны в нашей программе. Можно импортировать и только определенные функции и переменные из модуля.
Ввод и вывод данных
Переменной в программировании называется именованный контейнер для некоторого значения. Представьте себе ящик с наклееным на него стикером. Вы кладете что-нибудь в ящик и пишете на стикере короткое имя без пробелов и знаков препинания, начинающееся с буквы английского алфавита, примерно так же работает переменная в программировании.
a и А — это разные переменные, регистр ввода имеет значение
Типы данных
Информация получаемая нами с помощью различных органов чувств делится на зрительную, слуховую, обонятельную, осязательную и другие. В программировании так же есть свое разделение, разделение на типы данных. Примитивных типов данных в Python 4:
int — целочисленный тип (1, 192, 781287)
float — вещественный (дробный) (1.12312, 1231.12378718)
str — строковый тип (обязательно пишется в кавычках) (‘Hello world’)
bool — булевый тип (имеет всего два значения: True и False)
Приведение типов
Приведением типов данных называется преобразование одного типа в другой, например, строку в число, число в строку, число в булеву переменную, строку в дробь и так далее.
a = 10
b = str (a) # b — это строка
с = int (b) # c — это снова число
d = 10.78
e = int (d) # e равно 10
Функция print
Функция print выводит переданные в неё аргументы в стандартный поток вывода. Что же такое стандартный поток вывода? Standart output или stdout называется потоком вывода, местом, куда мы выводим наш текстовый контент. По умолчанию стандартный поток вывода равен sys.stdout и поэтому вывод осуществляется в консоль.
Функция print все переданные в неё аргументы в стандартный поток вывода. Например:
print ( 1 )
print ( ‘Hello world!’ )
print ( False )
print ( 1.5 , 2.0 , 10 , True , ‘username’ )
print ( ‘Hello’ + ‘ ‘ + ‘world’ + ‘!’ )
# 1
# Hello world!
# False
# 1.5 2.0 10 True username
# Hello world!
На этом тривиальном примере мы видим две вещи. Во-первых, каждый print переносит поток вывода на новую строчку. Во-вторых, аргументы, переданные в функцию print выводятся через пробел.
# Во-первых:
print ( 1 )
print ( 2 )
# 1
# 2
# Во-вторых:
print ( ‘Hello’ , ‘world’ )
# Hello world
В обоих случаях мы можем изменить стандартное поведение. Рассмотрим первый параметр функции print — end, в него передается строка, которая напечатается после всех аргументов функции print.
print ( 1 , end = ‘ ‘ )
print ( 2 , end = ‘ ‘ )
print ( 3 , end = ‘ ‘ )
# 1 2 3
print ( 1 , end = » — » )
print ( 2 , end = ‘-‘ )
print ( 3 , end = » — » )
# 1-2-3-
print ( 1 , end = ‘-я выведусь после первого print-‘ )
print ( 2 , end = ‘-a я после второго-‘ )
print ( 3 , end = ‘-я выведусь после третьего-‘ )
# 1-я выведусь после первого print-2-a я после второго-3-я выведусь после третьего-
Рассмотрим второй параметр функции print — sep, sep от английского separator (разделитель). По умолчанию параметр sep равен ‘ ‘. Время для экспериментов ╰(▔∀▔)╯.
print ( 1 , 2 , 3 , 4 )
print ( 1 , 2 , 3 , 4 , sep = ‘+++’ )
print ( 1 , 2 , 3 , 4 , sep = ‘разделитель’ )
print ( 1 , 2 , 3 , 4 , sep = ‘(◕‿◕)’ )
print ( 1 , 2 , 3 , 4 , sep = ‘(ﾉ◕ヮ◕)ﾉ*:･ﾟ✧’ )
# 1 2 3 4
# 1+++2+++3+++4
# 1разделитель2разделитель3разделитель4
# 1(◕‿◕)2(◕‿◕)3(◕‿◕)4
# 1(ﾉ◕ヮ◕)ﾉ*:･ﾟ✧2(ﾉ◕ヮ◕)ﾉ*:･ﾟ✧3(ﾉ◕ヮ◕)ﾉ*:･ﾟ✧4
Функция input
Функция input является функцией стандартного ввода (stdin). Ее главная задача — это передача введенных пользователем данных в функцию.
name = input ()
print ( ‘Hello, ‘ + name)
Функция input может принимать всего лишь один аргумент — строку, которая выведется перед кареткой ввода
name = input ( ‘Enter your name: ‘ )
print ( ‘Hello, ‘ + name)
Функция input возвращает строковый тип данных
Строки можно складывать друг с другом — это называется конкатенацией или объединением
number = input ()
print (type(number))
#
Поэтому если мы напишем такой код, то он будет работать некорректно:
number1 = input ()
number2 = input ()
print (number1 + number2)
# Ввод:
# 1
# 2
# Вывод:
# 12
Поэтому необходимо преобразовать строковый тип в целочисленный (str в int)
number1 = int ( input ())
number2 = int ( input ())
print (number1 + number2)
# Ввод:
# 1
# 2
# Вывод:
# 3
Всегда проверяйте тип полученных данных, это поможет вам избежать большого количества ошибок. К слову, если вы что-то напутали с типами переменных, то Python выдаст ошибку TypeError (ошибка типа)
Решение задач
1. Поэкспериментируйте с переводом в различные типы данных
2. Пользователь вводит свое имя и фамилию. Выведите:
Hello, имя фамилия
# На месте слов с % должны быть введенные данные
3. Посчитайте сумму трех введенных целых чисел
4. Посчитайте сумму трех введенных дробных чисел. Подумайте в какой тип данных нужно преобразовать значение, возвращенное функцией input
5. Дано число, выведите предыдущее и следущее за ним числа в таком формате:
# Число равно 10
Число предшествующее числу 10 равно 9
Число следующее за числом 10 равно 11
6. Вводятся имя и возраст. Выведите, где введенное имя = Максим, а возраст = 20