Python write file/ read file

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The file Object Attributes

Once a file is opened and you have one file object, you can get various information related to that file.


Here is a list of all attributes related to file object −

Sr.No. Attribute & Description
1

file.closed

Returns true if file is closed, false otherwise.

2

file.mode

Returns access mode with which file was opened.

3

file.name

Returns name of the file.

4

file.softspace

Returns false if space explicitly required with print, true otherwise.

Example

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "wb")
print "Name of the file: ", fo.name
print "Closed or not : ", fo.closed
print "Opening mode : ", fo.mode
print "Softspace flag : ", fo.softspace

This produces the following result −

Name of the file:  foo.txt
Closed or not :  False
Opening mode :  wb
Softspace flag :  0

Python File Reading Methods

  • file.read(n) – This method reads n number of characters from the file, or if n is blank it reads the entire file.
  • file.readline(n) – This method reads an entire line from the text file.

Example

f = open("test.txt","r") #opens file with name of "test.txt"
print(f.read(1))
print(f.read())

Result

I am a test file.Maybe someday, he will promote me to a real file.Man, I long to be a real fileand hang out with all my new real file friends.

Woot woot! So, this might be a little confusing. First, we open the file like expected. Next, we use the read(1), and notice that we provided an argument of 1, which just means we want to read the next character. So, Python prints out “I” for us because that is the first character in the test.txt file. What happens next is a little funky. We tell Python to read the entire file with read() because we did not provide any arguments. But, it doesn’t include that “I” that we just read!? It is because Python just picks up where it left off. So, if Python already read “I”, it will start reading from the next character. The reason for this is so you can cycle through a file without have to skip so many characters each time you want to read a new character. It’s complicated, I know, but think of reading like a one way process. Python is lazy, it doesn’t want to go back and reread contents. Let’s take this one step further with reading lines.

Example

f = open("test.txt","r") #opens file with name of "test.txt"
print(f.readline())
print(f.readline())

Result

I am a test file.Maybe someday, he will promote me to a real file.

Ha! This time we were prepared for Python’s trickery. Since we just used the readline() method twice, we knew that we would get first 2 lines because of Python’s reading process. Of course, we also knew that readline() reads only a line because of it’s simple syntax. Alright, one last important, complicated, and awesome thing about Python’s reading abilities.

Example

f = open("test.txt","r") #opens file with name of "test.txt"
myList = []
for line in f:
    myList.append(line)
print(myList)

Result

What!? Python just blew our minds! First, don’t freak out with the \n. It is just a newline character, since those lines are on a different file, Python wants to keep that format (you can always strip them out later).We open the file like normal, and we create a list, which we have mastered by now. Then, we break the file into lines in our for loop using the in keyword. Python is magically smart enough to understand that it should break files into lines. Next, as we are looping through each line of the file we use myList.append(line) to add each line to our myList list. Finally, when we print it out, Python shows us its glory. It broke each line of the file into a string, which we can manipulate to do whatever we want.

Writing to file

There are two ways to write in a file.

  1. write() : Inserts the string str1 in a single line in the text file.
    File_object.write(str1)
    
  2. writelines() : For a list of string elements, each string is inserted in the text file. Used to insert multiple strings at a single time.
    File_object.writelines(L) for L =  
    

Note: is treated as a special character of two bytes.

Example:

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Output:

Hello
This is Delhi
This is Paris
This is London

Appending to a file

When the file is opened in append mode, the handle is positioned at the end of the file. The data being written will be inserted at the end, after the existing data. Let’s see the below example to clarify the difference between write mode and append mode.

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Output:

Output of Readlines after appending
This is Delhi
This is Paris
This is London
Today


Output of Readlines after writing
Tomorrow

With statement

statement in Python is used in exception handling to make the code cleaner and much more readable. It simplifies the management of common resources like file streams. Unlike the above implementations, there is no need to call when using with statement. The statement itself ensures proper acquisition and release of resources.

Syntax:

with open filename as file:

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Output:

Hello
This is Delhi
This is Paris
This is London

Note: To know more about with statement click here.


My Personal Notes arrow_drop_up Save

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  • Open a File in Python
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Append Text to a File

The «w» mode will always treat the file as a new file. In other words, an existing file opened with «w» mode will lose its earlier contents. In order to add more data to existing file use the «a» or «a+» mode.

Example: Append and Read a File Copy

Opening a file with «w» mode or «a» mode can only be written into and cannot be read from. Similarly «r» mode allows reading only and not writing. In order to perform simultaneous read/append operations, use «a+» mode.

seek() method

To read or write at a specific position, use the function to set the current read/write position.

Here, the from parameter takes the following values:

  • 0 : offset calculated from the beginning
  • 1 : offset calculated from the current position
  • 2 : offset calculated from the end

Assuming that myfile.txt contains «Hello World» text, the following example demonstrates the method.

Example: seek() Copy

The output of the above example is «World».

File Positions

The tell() method tells you the current position within the file; in other words, the next read or write will occur at that many bytes from the beginning of the file.

The seek(offset) method changes the current file position. The offset argument indicates the number of bytes to be moved. The from argument specifies the reference position from where the bytes are to be moved.

If from is set to 0, it means use the beginning of the file as the reference position and 1 means use the current position as the reference position and if it is set to 2 then the end of the file would be taken as the reference position.

Example

Let us take a file foo.txt, which we created above.

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "r+")
str = fo.read(10)
print "Read String is : ", str

# Check current position
position = fo.tell()
print "Current file position : ", position

# Reposition pointer at the beginning once again
position = fo.seek(0, 0);
str = fo.read(10)
print "Again read String is : ", str
# Close opend file
fo.close()

This produces the following result −

Read String is :  Python is
Current file position :  10
Again read String is :  Python is

Mode

Including a mode argument is optional because a default value of ‘r’ will be assumed if it is omitted. The ‘r’ value stands for read mode, which is just one of many.

The modes are:

  • ‘r’ – Read mode which is used when the file is only being read
  • ‘w’ – Write mode which is used to edit and write new information to the file (any existing files with the same name will be erased when this mode is activated)
  • ‘a’ – Append mode, which is used to add new data to the end of the file; that is new information is automatically amended to the end
  • ‘r+’ – Special read and write mode, which is used to handle both actions when working with a file

So, let’s take a look at a quick example where we write file named “workfile” using the python open method.

This snippet opens the file named “workfile” in writing mode so that we can make changes to it.  Be sure you have the correct file path. The current information stored within the file is also displayed – or printed – for us to view.

Once this has been done, you can move on to call the objects functions. The two most common functions are read and write.

File Types

What you may know as a file is slightly different in Python.

In Windows, for example, a file can be any item manipulated, edited or created by the user/OS. That means files can be images, text documents, executables, and excel file and much more. Most files are organized by keeping them in individual folders.

In Python, a file is categorized as either text or binary, and the difference between the two file types is important.

Text files are structured as a sequence of lines, where each line includes a sequence of characters. This is what you know as code or syntax.

Each line is terminated with a special character, called the EOL or End of Line character. There are several types, but the most common is the comma {,} or newline character. It ends the current line and tells the interpreter a new one has begun.

A backslash character can also be used, and it tells the interpreter that the next character – following the slash – should be treated as a new line. This character is useful when you don’t want to start a new line in the text itself but in the code.

A binary file is any type of file that is not a text file. Because of their nature, binary files can only be processed by an application that know or understand the file’s structure. In other words, they must be applications that can read and interpret binary.

Reading a File

Given below is the simplest function to read a single character from a file −

int fgetc( FILE * fp );

The fgetc() function reads a character from the input file referenced by fp. The return value is the character read, or in case of any error, it returns EOF. The following function allows to read a string from a stream −

char *fgets( char *buf, int n, FILE *fp );

The functions fgets() reads up to n-1 characters from the input stream referenced by fp. It copies the read string into the buffer buf, appending a null character to terminate the string.

If this function encounters a newline character ‘\n’ or the end of the file EOF before they have read the maximum number of characters, then it returns only the characters read up to that point including the new line character. You can also use int fscanf(FILE *fp, const char *format, …) function to read strings from a file, but it stops reading after encountering the first space character.

#include <stdio.h>

main() {

   FILE *fp;
   char buff;

   fp = fopen("/tmp/test.txt", "r");
   fscanf(fp, "%s", buff);
   printf("1 : %s\n", buff );

   fgets(buff, 255, (FILE*)fp);
   printf("2: %s\n", buff );
   
   fgets(buff, 255, (FILE*)fp);
   printf("3: %s\n", buff );
   fclose(fp);

}

When the above code is compiled and executed, it reads the file created in the previous section and produces the following result −

1 : This
2: is testing for fprintf...

3: This is testing for fputs...

Let’s see a little more in detail about what happened here. First, fscanf() read just This because after that, it encountered a space, second call is for fgets() which reads the remaining line till it encountered end of line. Finally, the last call fgets() reads the second line completely.

Задания для самоподготовки

1. Выполните считывание данных из текстового файла через символ и записи прочитанных данных в другой текстовый файл. Прочитывайте так не более 100 символов.

2. Пользователь вводит предложение с клавиатуры. Разбейте это предложение по словам (считать, что слова разделены пробелом) и сохраните их в столбец в файл.

3. Пусть имеется словарь:

d = {«house»: «дом», «car»: «машина»,

     «tree»: «дерево», «road»: «дорога»,

     «river»: «река»}

Необходимо каждый элемент этого словаря сохранить в бинарном файле как объект. Затем, прочитать этот файл и вывести считанные объекты в консоль.

Видео по теме

Python 3 #1: установка и запуск интерпретатора языка

Python 3 #2: переменные, оператор присваивания, типы данных

Python 3 #3: функции input и print ввода/вывода

Python 3 #4: арифметические операторы: сложение, вычитание, умножение, деление, степень

Python 3 #5: условный оператор if, составные условия с and, or, not

Python 3 #6: операторы циклов while и for, операторы break и continue

Python 3 #7: строки — сравнения, срезы строк, базовые функции str, len, ord, in

Python 3 #8: методы строк — upper, split, join, find, strip, isalpha, isdigit и другие

Python 3 #9: списки list и функции len, min, max, sum, sorted

Python 3 #10: списки — срезы и методы: append, insert, pop, sort, index, count, reverse, clear

Python 3 #11: списки — инструмент list comprehensions, сортировка методом выбора

Python 3 #12: словарь, методы словарей: len, clear, get, setdefault, pop

Python 3 #13: кортежи (tuple) и операции с ними: len, del, count, index


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Python 3 #15: делаем «Сапер», проектирование программ «сверху-вниз»

Python 3 #16: рекурсивные и лямбда-функции, функции с произвольным числом аргументов

Python 3 #17: алгоритм Евклида, принцип тестирования программ

Python 3 #18: области видимости переменных — global, nonlocal

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Python 3 #22: сортировка sort() и sorted(), сортировка по ключам

Python 3 #23: обработка исключений: try, except, finally, else

Python 3 #24: файлы — чтение и запись: open, read, write, seek, readline, dump, load, pickle

Python 3 #25: форматирование строк: метод format и F-строки

Python 3 #26: создание и импорт модулей — import, from, as, dir, reload

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Запись информации в файл

Теперь давайте посмотрим, как происходит запись информации в файл. Во-первых, нам нужно открыть файл на запись, например, так:

file = open("out.txt", "w")

и далее вызвать метод write:

file.write("Hello World!")

В результате у нас будет создан файл out.txt со строкой «Hello World!». Причем, этот файл будет располагаться в том же каталоге, что и файл с текстом программы на Python.

Далее сделаем такую операцию: запишем метод write следующим образом:

file.write("Hello")

И снова выполним эту программу. Смотрите, в нашем файле out.txt прежнее содержимое исчезло и появилось новое – строка «Hello». То есть, когда мы открываем файл на запись в режимах

w, wt, wb,

то прежнее содержимое файла удаляется. Вот этот момент следует всегда помнить.

Теперь посмотрим, что будет, если вызвать метод write несколько раз подряд:

    file.write("Hello1")
    file.write("Hello2")
    file.write("Hello3")

Смотрите, у нас в файле появились эти строчки друг за другом. То есть, здесь как и со считыванием: объект file записывает информацию, начиная с текущей файловой позиции, и автоматически перемещает ее при выполнении метода write.

Если мы хотим записать эти строчки в файл каждую с новой строки, то в конце каждой пропишем символ переноса строки:

   file.write("Hello1\n")
   file.write("Hello2\n")
   file.write("Hello3\n")

Далее, для дозаписи информации в файл, то есть, записи с сохранением предыдущего содержимого, файл следует открыть в режиме ‘a’:

file = open("out.txt", "a")

Тогда, выполняя эту программу, мы в файле увидим уже шесть строчек. И смотрите, в зависимости от режима доступа к файлу, мы должны использовать или методы для записи, или методы для чтения. Например, если вот здесь попытаться прочитать информацию с помощью метода read:

file.read()

то возникнет ошибка доступа. Если же мы хотим и записывать и считывать информацию, то можно воспользоваться режимом a+:

file = open("out.txt", "a+")

Так как здесь файловый указатель стоит на последней позиции, то для считывания информации, поставим его в самое начало:

   file.seek()
   print( file.read() )

А вот запись данных всегда осуществляется в конец файла.

Следующий полезный метод для записи информации – это writelines:

file.writelines("Hello1\n", "Hello2\n")

Он записывает несколько строк, указанных в коллекции. Иногда это бывает удобно, если в процессе обработки текста мы имеем список и его требуется целиком поместить в файл.

Opening a File in Python

test.txt

I am a test file.
Maybe someday, he will promote me to a real file.
Man, I long to be a real file
and hang out with all my new real file friends.

Example

f = open("test.txt", "r") #opens file with name of "test.txt"

This is pretty simple to explain. We have our awesome little test.txt file filled with some random text. Now, in the example we have our code. Just like you click a file to open it, Python needs to know what file to open. So, we use the open method to tell Python what we want to open and to go ahead and open (make a connection to) it. The “r” just tells Python that we want to read (it is “w” when we want to write to a file). And, of course, we set this new connection to a variable so we can use it later. However, we have only opened a file, which is not all that exciting. Let’s try reading from the file.

Reading the file

Okay, you’ve opened the file, now how to read it?

The function returns a object and it supports Pythons iteration protocol. Each iteration will give you a line:

This will print each line of the file. Note however that each line will contain a newline character at the end (you might want to check if your Python is built with — otherwise you could also have on Windows or on Mac as newlines). If you don’t want that you can could simply remove the last character (or the last two characters on Windows):

But the last line doesn’t necessarily has a trailing newline, so one shouldn’t use that. One could check if it ends with a trailing newline and if so remove it:

But you could simply remove all whitespaces (including the character) from the end of the string, this will also remove all other trailing whitespaces so you have to be careful if these are important:

However if the lines end with (Windows «newlines») that will also take care of the !

The read() Method

The read() method reads a string from an open file. It is important to note that Python strings can have binary data. apart from text data.

Syntax

fileObject.read()

Here, passed parameter is the number of bytes to be read from the opened file. This method starts reading from the beginning of the file and if count is missing, then it tries to read as much as possible, maybe until the end of file.

Example

Let’s take a file foo.txt, which we created above.

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "r+")
str = fo.read(10);
print "Read String is : ", str
# Close opend file
fo.close()

This produces the following result −

Read String is :  Python is

FileReader

is an object with the sole purpose of reading data from (and hence too) objects.

It delivers the data using events, as reading from disk may take time.

The constructor:

The main methods:

  • – read the data in binary format .
  • – read the data as a text string with the given encoding ( by default).
  • – read the binary data and encode it as base64 data url.
  • – cancel the operation.

The choice of method depends on which format we prefer, how we’re going to use the data.

  • – for binary files, to do low-level binary operations. For high-level operations, like slicing, inherits from , so we can call them directly, without reading.
  • – for text files, when we’d like to get a string.
  • – when we’d like to use this data in for or another tag. There’s an alternative to reading a file for that, as discussed in chapter Blob: .

As the reading proceeds, there are events:

  • – loading started.
  • – occurs during reading.
  • – no errors, reading complete.
  • – called.
  • – error has occurred.
  • – reading finished with either success or failure.

When the reading is finished, we can access the result as:

  • is the result (if successful)
  • is the error (if failed).

The most widely used events are for sure and .

Here’s an example of reading a file:

for blobs

As mentioned in the chapter Blob, can read not just files, but any blobs.

We can use it to convert a blob to another format:

  • – to ,
  • – to string (an alternative to ),
  • – to base64 data url.

is available inside Web Workers

For Web Workers, there also exists a synchronous variant of , called .

Its reading methods do not generate events, but rather return a result, as regular functions do.


That’s only inside a Web Worker though, because delays in synchronous calls, that are possible while reading from files, in Web Workers are less important. They do not affect the page.

Write to an Existing File

To write to an existing file, you must add a parameter to the function:

— Append — will append to the end of the file

— Write — will overwrite any existing content

Example

Open the file «demofile2.txt» and append content to the file:

f = open(«demofile2.txt», «a»)f.write(«Now the file has more content!») f.close()#open and read the file after the appending:f = open(«demofile2.txt», «r»)print(f.read())

Example

Open the file «demofile3.txt» and overwrite the content:

f = open(«demofile3.txt», «w») f.write(«Woops! I have deleted the content!»)f.close() #open and read the file after the appending:f = open(«demofile3.txt», «r») print(f.read())

Note: the «w» method will overwrite the entire file.

With Statement

You can also work with file objects using the with statement. It is designed to provide much cleaner syntax and exceptions handling when you are working with code. That explains why it’s good practice to use the with statement where applicable.

One bonus of using this method is that any files opened will be closed automatically after you are done. This leaves less to worry about during cleanup.

To use the with statement to open a file:

Now that you understand how to call this statement, let’s take a look at a few examples.

You can also call upon other methods while using this statement. For instance, you can do something like loop over a file object:

You’ll also notice that in the above example we didn’t use the “file.close()” method because the with statement will automatically call that for us upon execution. It really makes things a lot easier, doesn’t it?

Parameters

A handle to the device (for example, a file, file stream, physical disk, volume, console buffer, tape drive, socket, communications resource, mailslot, or pipe).

The hFile parameter must have been created with read access. For more information, see Generic Access Rights and File Security and Access Rights.

For asynchronous read operations, hFile can be any handle that is opened with the FILE_FLAG_OVERLAPPED flag by the CreateFile function, or a socket handle returned by the socket or accept function.

A pointer to the buffer that receives the data read from a file or device.

This buffer must remain valid for the duration of the read operation. The caller must not use this buffer until the read operation is completed.

The maximum number of bytes to be read.

A pointer to the variable that receives the number of bytes read when using a synchronous hFile parameter. ReadFile sets this value to zero before doing any work or error checking. Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.

This parameter can be NULL only when the lpOverlapped parameter is not NULL.

For more information, see the Remarks section.

A pointer to an OVERLAPPED structure is required if the hFile parameter was opened with FILE_FLAG_OVERLAPPED, otherwise it can be NULL.

If hFile is opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the read operation is complete.

For an hFile that supports byte offsets, if you use this parameter you must specify a byte offset at which to start reading from the file or device. This offset is specified by setting the Offset and OffsetHigh members of the OVERLAPPED structure. For an hFile that does not support byte offsets, Offset and OffsetHigh are ignored.

For more information about different combinations of lpOverlapped and FILE_FLAG_OVERLAPPED, see the Remarks section and the Synchronization and File Position section.

The write() Method

The write() method writes any string to an open file. It is important to note that Python strings can have binary data and not just text.

The write() method does not add a newline character (‘\n’) to the end of the string −

fileObject.write(string)

Here, passed parameter is the content to be written into the opened file.

Example

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "wb")
fo.write( "Python is a great language.\nYeah its great!!\n")

# Close opend file
fo.close()

The above method would create foo.txt file and would write given content in that file and finally it would close that file. If you would open this file, it would have following content.

Python is a great language.
Yeah its great!!

Python NumPy

NumPy IntroNumPy Getting StartedNumPy Creating ArraysNumPy Array IndexingNumPy Array SlicingNumPy Data TypesNumPy Copy vs ViewNumPy Array ShapeNumPy Array ReshapeNumPy Array IteratingNumPy Array JoinNumPy Array SplitNumPy Array SearchNumPy Array SortNumPy Array FilterNumPy Random Random Intro Data Distribution Random Permutation Seaborn Module Normal Distribution Binomial Distribution Poisson Distribution Uniform Distribution Logistic Distribution Multinomial Distribution Exponential Distribution Chi Square Distribution Rayleigh Distribution Pareto Distribution Zipf Distribution

NumPy ufunc ufunc Intro ufunc Create Function ufunc Simple Arithmetic ufunc Rounding Decimals ufunc Logs ufunc Summations ufunc Products ufunc Differences ufunc Finding LCM ufunc Finding GCD ufunc Trigonometric ufunc Hyperbolic ufunc Set Operations

Opening the file

I assume that you want to open a specific file and you don’t deal directly with a file-handle (or a file-like-handle). The most commonly used function to open a file in Python is , it takes one mandatory argument and two optional ones in Python 2.7:

  • Filename
  • Mode
  • Buffering (I’ll ignore this argument in this answer)

The filename should be a string that represents the path to the file. For example:

Note that the file extension needs to be specified. This is especially important for Windows users because file extensions like or , etc. are hidden by default when viewed in the explorer.

The second argument is the , it’s by default which means «read-only». That’s exactly what you need in your case.

But in case you actually want to create a file and/or write to a file you’ll need a different argument here. There is an excellent answer if you want an overview.

For reading a file you can omit the or pass it in explicitly:

Both will open the file in read-only mode. In case you want to read in a binary file on Windows you need to use the mode :

On other platforms the (binary mode) is simply ignored.

Now that I’ve shown how to the file, let’s talk about the fact that you always need to it again. Otherwise it will keep an open file-handle to the file until the process exits (or Python garbages the file-handle).

While you could use:

That will fail to close the file when something between and throws an exception. You could avoid that by using a and :

However Python provides context managers that have a prettier syntax (but for it’s almost identical to the and above):

The last approach is the recommended approach to open a file in Python!

Python NumPy

NumPy IntroNumPy Getting StartedNumPy Creating ArraysNumPy Array IndexingNumPy Array SlicingNumPy Data TypesNumPy Copy vs ViewNumPy Array ShapeNumPy Array ReshapeNumPy Array IteratingNumPy Array JoinNumPy Array SplitNumPy Array SearchNumPy Array SortNumPy Array FilterNumPy Random Random Intro Data Distribution Random Permutation Seaborn Module Normal Distribution Binomial Distribution Poisson Distribution Uniform Distribution Logistic Distribution Multinomial Distribution Exponential Distribution Chi Square Distribution Rayleigh Distribution Pareto Distribution Zipf Distribution

NumPy ufunc ufunc Intro ufunc Create Function ufunc Simple Arithmetic ufunc Rounding Decimals ufunc Logs ufunc Summations ufunc Products ufunc Differences ufunc Finding LCM ufunc Finding GCD ufunc Trigonometric ufunc Hyperbolic ufunc Set Operations

Splitting Lines in a Text File

As a final example, let’s explore a unique function that allows you to split the lines taken from a text file. What this is designed to do, is split the string contained in variable data whenever the interpreter encounters a space character.

But just because we are going to use it to split lines after a space character, doesn’t mean that’s the only way. You can actually split your text using any character you wish – such as a colon, for instance.

The code to do this (also using a with statement) is:

If you wanted to use a colon instead of a space to split your text, you would simply change line.split() to line.split(“:”).

The output for this will be:

The reason the words are presented in this manner is because they are stored – and returned – as an array. Be sure to remember this when working with the split function.

Writing to a File

The following example writes to a physical file.

Example: Writing to File Copy

In the above example, the statement opens in write mode, the method returns the file object and assigns it to a variable . «w» specifies that the file should be writable. Next, we have to put certain data in the file. The stores a string in the file. In the end, closes the file object.

When you run the above code, you will find «myfile.txt» created on the D drive of your computer. You can see the contents by opening it with an editor, like Notepad.

Python provides the method to save the contents of a list object in a file. Since the newline character is not automatically written to the file, it must be provided as a part of the string.

Example: Write Lines to File Copy


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