Every open file has an implicit pointer which indicates where data will be read and written. Normally this defaults to the start of the file, but if you use a mode of a
[append] then it defaults to the end of the file. It's also worth noting that the w
mode will truncate your file [i.e. delete all the contents] even if you add +
to the mode.
Whenever you read or write N characters, the read/write pointer will move forward that amount within the file. I find it helps to think of this like an old cassette tape, if you remember those. So, if you executed the following code:
fd = open["testfile.txt", "w+"]
fd.write["This is a test file.\n"]
fd.close[]
fd = open["testfile.txt", "r+"]
print fd.read[4]
fd.write[" IS"]
fd.close[]
... It should end up printing This
and then leaving the file content as This IS a test file.
. This is because the initial read[4]
returns the first 4 characters of the file, because the pointer is at the start of the file. It leaves the pointer at the space character just after This
, so the following write[" IS"]
overwrites the next three characters with a space
[the same as is already there] followed by IS
, replacing the existing is
.
You can use the seek[]
method of the file to jump to a specific point. After the example above, if you executed the following:
fd = open["testfile.txt", "r+"]
fd.seek[10]
fd.write["TEST"]
fd.close[]
... Then you'll find that the file now contains This IS a TEST file.
.
All this applies on Unix systems, and you can test those examples to make sure. However, I've had problems mixing read[]
and write[]
on Windows systems. For example, when I execute that first example
on my Windows machine then it correctly prints This
, but when I check the file afterwards the write[]
has been completely ignored. However, the second example [using seek[]
] seems to work fine on Windows.
In summary, if you want to read/write from the middle of a file in Windows I'd suggest always using an explicit seek[]
instead of relying on the position of the read/write pointer. If you're doing only reads or only writes then it's pretty safe.
One final point - if you're specifying paths on Windows as literal strings, remember to escape your backslashes:
fd = open["C:\\Users\\johndoe\\Desktop\\testfile.txt", "r+"]
Or you can use raw strings by putting an r
at the start:
fd = open[r"C:\Users\johndoe\Desktop\testfile.txt", "r+"]
Or the most portable option is to use os.path.join[]
:
fd = open[os.path.join["C:\\", "Users", "johndoe", "Desktop", "testfile.txt"], "r+"]
You can find more information about file IO in the official Python docs.
Python provides inbuilt functions for creating, writing, and reading files. There are two types of files that can be handled in python, normal text files and binary files [written in binary language, 0s, and 1s].
- Text files: In this type of file, Each line of text is terminated with a special character called EOL [End of Line], which is the new line character [‘\n’] in python by default.
- Binary files: In this type of file, there is no terminator for a line, and the data is stored after converting it into machine-understandable binary language.
In this article, we will be focusing on opening, closing, reading, and writing data in a text file.
File Access Modes
Access modes govern the type of operations possible in the opened file. It refers to how the file will be used once its opened. These modes also define the location of the File Handle in the file. File handle is like a cursor, which defines from where the data has to be read or written in the file. There are 6 access modes in python.
- Read Only [‘r’] : Open text file for reading. The handle is positioned at the beginning of the file. If the file does not exists, raises the I/O error. This is also the default mode in which a file is opened.
- Read and Write [‘r+’]: Open the file for reading and writing. The handle is positioned at the beginning of the file. Raises I/O error if the file does not exist.
- Write Only [‘w’] : Open the file for writing. For the existing files, the data is truncated and over-written. The handle is positioned at the beginning of the file. Creates the file if the file does not exist.
- Write and Read [‘w+’] : Open the file for reading and writing. For an existing file, data is truncated and over-written. The handle is positioned at the beginning of the file.
- Append Only [‘a’]: Open the file for writing. The file is created if it does not exist. The handle is positioned at the end of the file. The data being written will be inserted at the end, after the existing data.
- Append and Read [‘a+’] : Open the file for reading and writing. The file is created if it does not exist. The handle is positioned at the end of the file. The data being written will be inserted at the end, after the existing data.
How Files are Loaded into Primary Memory
There are two kinds of memory in a computer i.e. Primary and Secondary memory every file that you saved or anyone saved is on secondary memory cause any data in primary memory is deleted when the computer is powered off. So when you need to change any text file or just to work with them in python you need to load that file into primary memory. Python interacts with files loaded in primary memory or main memory through “file handlers” [ This is how your operating system gives access to python to interact with the file you opened by searching the file in its memory if found it returns a file handler and then you can work with the file ].
Opening a File
It is done using the open[] function. No module is required to be imported for this function.
File_object = open[r"File_Name","Access_Mode"]
The file should exist in the same directory as the python program file else, the full address of the file should be written in place of the filename. Note: The r is placed before the filename to prevent the characters in the filename string to be treated as special characters. For example, if there is \temp in the file address, then \t is treated as the tab character, and an error is raised of invalid address. The r makes the string raw, that is, it tells that the string is without any special characters. The r can be ignored if the file is in the same directory and the address is not being placed.
Python
file1
=
open
[
"MyFile.txt"
,
"a"
]
file2
=
open
[r
"D:\Text\MyFile2.txt"
,
"w+"
]
Here, file1 is created as an object for MyFile1 and file2 as object for MyFile2
Closing a file
close[] function closes the file and frees the memory space acquired by that file. It is used at the time when the file is no longer needed or if it is to be opened in a different file mode. File_object.close[]
Python
file1
=
open
[
"MyFile.txt"
,
"a"
]
file1.close[]
Writing to a file
There are two ways to write in a file.
- write[] : Inserts the string str1 in a single line in the text file.
File_object.write[str1]
- 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 = [str1, str2, str3]
Reading from a file
There are three ways to read data from a text file.
- read[] : Returns the read bytes in form of a string. Reads n bytes, if no n specified, reads the entire file.
File_object.read[[n]]
- readline[] : Reads a line of the file and returns in form of a string.For specified n, reads at most n bytes. However, does not reads more than one line, even if n exceeds the length of the line.
File_object.readline[[n]]
- readlines[] : Reads all the lines and return them as each line a string element in a list.
File_object.readlines[]
Note: ‘\n’ is treated as a special character of two bytes
Python3
file1
=
open
[
"myfile.txt"
,
"w"
]
L
=
[
"This is Delhi \n"
,
"This is Paris \n"
,
"This is London \n"
]
file1.write[
"Hello \n"
]
file1.writelines[L]
file1.close[]
file1
=
open
[
"myfile.txt"
,
"r+"
]
print
[
"Output of Read function is "
]
print
[file1.read[]]
print
[]
file1.seek[
0
]
print
[
"Output of Readline function is "
]
print
[file1.readline[]]
print
[]
file1.seek[
0
]
print
[
"Output of Read[9] function is "
]
print
[file1.read[
9
]]
print
[]
file1.seek[
0
]
print
[
"Output of Readline[9] function is "
]
print
[file1.readline[
9
]]
file1.seek[
0
]
print
[
"Output of Readlines function is "
]
print
[file1.readlines[]]
print
[]
file1.close[]
Output:
Output of Read function is Hello This is Delhi This is Paris This is London Output of Readline function is Hello Output of Read[9] function is Hello Th Output of Readline[9] function is Hello Output of Readlines function is ['Hello \n', 'This is Delhi \n', 'This is Paris \n', 'This is London \n']
Appending to a file
Python3
file1
=
open
[
"myfile.txt"
,
"w"
]
L
=
[
"This is Delhi \n"
,
"This is Paris \n"
,
"This is London \n"
]
file1.writelines[L]
file1.close[]
file1
=
open
[
"myfile.txt"
,
"a"
]
file1.write[
"Today \n"
]
file1.close[]
file1
=
open
[
"myfile.txt"
,
"r"
]
print
[
"Output of Readlines after appending"
]
print
[file1.readlines[]]
print
[]
file1.close[]
file1
=
open
[
"myfile.txt"
,
"w"
]
file1.write[
"Tomorrow \n"
]
file1.close[]
file1
=
open
[
"myfile.txt"
,
"r"
]
print
[
"Output of Readlines after writing"
]
print
[file1.readlines[]]
print
[]
file1.close[]
Output:
Output of Readlines after appending ['This is Delhi \n', 'This is Paris \n', 'This is London \n', 'Today \n'] Output of Readlines after writing ['Tomorrow \n']
Related Article: File Objects in Python
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