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.
Sep в python как использовать
The separator between the arguments to print() function in Python is space by default (softspace feature) , which can be modified and can be made to any character, integer or string as per our choice. The ‘sep’ parameter is used to achieve the same, it is found only in python 3.x or later. It is also used for formatting the output strings.
Examples:
Python3
Output:
The sep parameter when used with the end parameter it produces awesome results. Some examples by combining the sep and end parameters.
Python3
Output:
Note: Please change the language from Python to Python 3 in the online ide.
Go to your interactive python ide by typing python in your cmd ( windows ) or terminal ( linux )
Python3
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using the sep parameter in the print() function:
By default, the sep parameter is set to a space character, so if you don’t specify it explicitly, the values will be separated by a space.
Approach:
The code is using the print() function to print out strings with different separators. The sep parameter of the print() function is used to specify the separator between the strings. In the first example, a comma is used as the separator, in the second example, a semicolon is used, and in the third example, an emoji is used.
Time Complexity:
The time complexity of the print() function is O(n), where n is the total number of characters to be printed. However, the time complexity of specifying a separator is O(1), as it is a constant time operation.
Space Complexity:
The space complexity of the code is also O(n), where n is the total number of characters to be printed. This is because the print() function needs to allocate memory to store the strings and separators before printing them out.
Overall, the code has a constant time complexity for specifying the separator, and a linear time and space complexity for printing out the strings and separators.
The ‘sep’ and ‘end’ parameters in Python print statement
We all have written multiple print statements in Python with or without parameters. But have you ever wondered about the significance of using sep and end parameters to the print statement?
In this post, we will discuss how ‘sep’ and ‘end’ parameters can be used to change the way in which the contents of the print method is printed on the console.
What are the Python sep and end Parameters in Python?
The end parameter in Print is used to append any string at the end of the output of the print statement in python while the sep parameter is used to format the strings that need to be printed on the console and add a separator between strings.
Let’s learn more about these Python print parameters!
1. The end parameter
The end parameter is used to append any string at the end of the output of the print statement in python.
By default, the print the method ends with a new line. This means there is no need to explicitly specify the parameter end as ‘\n’. Programmers with a background in C/C++ may find this interesting.
Let us look at how changing the value of the end parameter changes the contents of the print statement onscreen.
The below example demonstrates that passing ‘\n’ or not specifying the end parameter both yield the same result. Execute 2 lines of code at a time to see the result.
Output:
On the other hand, passing the whitespace to the end parameter indicates that the end character has to be identified by whitespace and not a new line (which is the default).
Output:
The below example shows that any value can be passed to the end parameter and based on the content in the print statement, the end value gets displayed.
Output:
Learn Python Language from Basic to Advanced Level
2. The sep parameter
Sometimes, we may wish to print multiple values in a Python program in a readable manner. This is when the argument sep comes to play. The arguments passed to the program can be separated by different values. The default value for sep is whitespace. The sep parameter is primarily used to format the strings that need to be printed on the console and add a separator between strings to be printed. This feature was newly introduced in Python 3 version.
The below example shows that passing the sep parameter as whitespace or not passing the sep at all doesn’t make a difference. Execute every line of code to see the result.
Output:
The below example shows different values that are passed to the sep parameter.
Output:
Note: The sep parameter, used in conjunction with the end parameter is generally used in production code to print data in a readable fashion.
Output:
Conclusion:
In this tutorial, We discussed the sep and end parameters in the print statements of Python then we discussed both of them in detail and executed examples of both. You can learn awesome stuff like this for Free at StudyTonight. We offer a beginner to advance level Python Course that can help you grow your Python skills in a very fun and interactive environment!
Как работает функция print в Python
Функция print() в Python используется для вывода текстовой информации на экран или в консоль. Эта функция может принимать один или несколько аргументов. Одним из обязательных аргументов является строка или объект, который будет выведен.
Вы наверняка знакомы с этой функцией. С неё начинается любой урок по программированию на Python.
В данном случае функция выведет в консоль сообщение:
При этом обязательно использовать скобки, иначе возникнет синтаксическая ошибка. В Python 2 скобки можно было не указывать, но в Python 3 скобки необходимы. Всё потому, что в третьей версии print() — это функция, а не инструкция.
Убедимся, что перед нами действительно функция, и спросим Python:
Python ответит нам, что перед нами встроенная функция или метод.
Аргументы print() в Python
Аргументы функции print() позволяют изменять способ форматирования сообщений и управлять стилем вывода.
Вот полный синтаксис функции, скрытый от наших глаз:
Аргументы функции здесь — это sep , end , file и flush :
- sep позволяет задать разделитель, который будет выводиться между элементами, переданными в функцию print(). По умолчанию разделителем является пробел, но с помощью sep пробел можно заменить на другой разделитель, к примеру, на запятую.
- end позволяет определять символ, который будет добавлен в конец сообщения после вывода. По умолчанию это символ перевода строки. Поменять его можно, к примеру, на точку с запятой.
- file позволяет перенаправить вывод текста в нужный вам файл. По умолчанию функция выводит текст в консоль, в которой вы и задаёте команды Python. Аргумент file полезен, если нужно сохранить вывод для дальнейшего использования.
- flush позволяет управлять буферизацией вывода. Если этот аргумент установлен в True, то вывод не будет буферизоваться и будет выводиться немедленно.
Рассмотрим эти аргументы чуть подробнее.
Аргумент sep в функции print() на Python
Как мы уже поняли, sep отвечает за то, какой символ будет отделять друг от друга элементы вывода. По умолчанию, если sep не поменять, этот символ будет пробелом.
Попросим Python вывести на экран слова, не меняя при этом sep .
Убеждаемся, что sep по умолчанию — это пробел. На экране появится такое сообщение:
Теперь попробуем заменить дефолтный пробел в sep на что-то другое. Используем символы \n в качестве разделителя в аргументе sep . Они должны перенести каждый следующий элемент выдачи на новую строку.
Получается вот такой вывод функции:
Как и ожидалось, после каждого слова в выводе строка переносится, ведь разделителем мы назначили именно перенос строки. ��
Аргумент end в функции print() на Python
Аргумент end — это суть то же самое, что и sep . Разница только в том, что sep — это разделитесь между элементами вывода, а end — это разделитесь самих выводов разных функций print() , если их несколько.
По умолчанию end — это перенос строки. Его тоже можно заменить на любое другое удобное значение.
Вот, как выглядит стандартный вывод функций print() , если не менять аргумент end .
Попробуем заменить перенос строки в аргументе end на обычный пробел.
Вывод с обычным пробелом вместо переноса строки:
Аргумент file в функции print() на Python
Аргумент file позволяет записывать данные вывода в файлы с расширением txt или csv.
По умолчанию значение аргумента file указано как sys.stdout . Это — вывод значений непосредственно в консоль Python.
Допустим, нам нужно, чтобы по завершении программа записала вывод в файлик print.txt. Для этого напишем простую программу, которая:
- откроет файл print.txt;
- запишет данные вывода в файл;
- закроет файл print.txt.
Результатом выполнения этого кода станет появление файла print.txt. Внутри него должно оказаться:
Аргумент flush в функции print() на Python
Аргумент flush в функции print() в Python позволяет управлять выводом сообщений без задержек.
Дело в том, что по умолчанию программа будет записывать данные вывода из print() сперва в буфер обмена, а уже потом выводить данные в консоль или в файл.
По умолчанию аргумент flush установлен в значении False .
Когда значение аргумента flush установлено в True , Python выводит сообщение, не дожидаясь буферизации вывода. Это полезно, когда нужно мгновенно показать вывод в консоли или в файле.
В этом примере мы записали вывод Hello, world! в файл output.txt без буферизации, установив значение аргумента flush как True .
Однако с этим аргументом нужно быть осторожным:
- Если возникнет ошибка в выполнении программы, а аргумент flush установлен как True, результат вывода может быть утерян. Он не сохранится в буфере из-за значения True и не отобразится в консоли или в файле из-за ошибки.
- Использование flush=True при работе с несколькими потоками может привести к тому, что разные потоки будут пытаться выводить данные в поток вывода одновременно. Это приведет к ошибкам.
Использование flush=True без должной осторожности может создать проблемы в работе программы и усложнить отладку. Если нет необходимости в ручной записи данных в поток вывода, лучше не использовать этот параметр.
В общем, аргументы функции print() позволяют управлять тем, как сообщения выводятся на экран и в консоль, и как они форматируются и выводятся. Это делает print() очень универсальной функцией, которую можно использовать во многих различных ситуациях.