What is python?
Python is an interpreted, object-oriented, high-level programming language with dynamic semantics. Its high-level built in data structures, combined with dynamic typing and dynamic binding, make it very attractive for Rapid Application Development, as well as for use as a scripting or glue language to connect existing components together. Python’s simple, easy to learn syntax emphasizes readability and therefore reduces the cost of program maintenance. Python supports modules and packages, which encourages program modularity and code reuse. The Python interpreter and the extensive standard library are available in source or binary form without charge for all major platforms, and can be freely distributed.
How to encrypt and decrypt a file
First you need to install the cryptography library.
To install cryptography open python terminal and type pip install cryptography and press enter
cryptography will then begin to installed.
Then you need to import cryptography into your script on line 1 type
from cryptography.fernet import Fernet
Then you will need to generate a encryption key to do the enter the following into your script
# key generation
key = Fernet.generate_key()
# string the key in a file
with open(‘filekey.key’, ‘wb’) as filekey:
filekey.write(key)
Now you can encrypt the file by adding
# opening the key
with open(‘filekey.key’, ‘rb’) as filekey:
key = filekey.read()
# using the generated key
fernet = Fernet(key)
# opening the original file to encrypt
with open(‘nba.csv’, ‘rb’) as file:
original = file.read()
# encrypting the file
encrypted = fernet.encrypt(original)
# opening the file in write mode and
# writing the encrypted data
with open(‘nba.csv’, ‘wb’) as encrypted_file:
encrypted_file.write(encrypted)
You have now encrypted the file to decrypt the file you need to use the encryption key using the code below will decrypt the file
# using the key
fernet = Fernet(key)
# opening the encrypted file
with open(‘nba.csv’, ‘rb’) as enc_file:
encrypted = enc_file.read()
# decrypting the file
decrypted = fernet.decrypt(encrypted)
# opening the file in write mode and
# writing the decrypted data
with open(‘nba.csv’, ‘wb’) as dec_file:
dec_file.write(decrypted)
File handling
Opening a file
Python has a built-in function open() to open a file. This function returns a file object, also called a handle, as it is used to read or modify the file accordingly.
>>> f = open(“test.txt”) # open file in current directory
>>> f = open(“C:/Python33/README.txt”) # specifying full path
We can specify the mode while opening a file. In mode, we specify whether we want to read ‘r’, write ‘w’ or append ‘a’ to the file. We also specify if we want to open the file in text mode or binary mode. The default is reading in text mode. In this mode, we get strings when reading from the file. On the other hand, binary mode returns bytes and this is the mode to be used when dealing with non-text files like image or exe files.
Mode Description
‘r’ Open a file for reading. (default)
‘w’ Open a file for writing. Creates a new file if it does not exist or truncates the file if it exists.
‘x’ Open a file for exclusive creation. If the file already exists, the operation fails.
‘a’ Open for appending at the end of the file without truncating it. Creates a new file if it does not exist.
‘t’ Open in text mode. (default)
‘b’ Open in binary mode.
‘+’ Open a file for updating (reading and writing)
f = open(“test.txt”) # equivalent to ‘r’ or ‘rt’
f = open(“test.txt”,’w’) # write in text mode
f = open(“img.bmp”,’r+b’) # read and write in binary mode
Unlike other languages, the character ‘a’ does not imply the number 97 until it is encoded using ASCII (or other equivalent encodings). Moreover, the default encoding is platform dependent. In windows, it is ‘cp1252’ but ‘utf-8’ in Linux. So, we must not also rely on the default encoding or else our code will behave differently in different platforms. Hence, when working with files in text mode, it is highly recommended to specify the encoding type.
f = open(“test.txt”,mode = ‘r’,encoding = ‘utf-8’)
Closing a file
When we are done with operations to the file, we need to properly close it. Closing a file will free up the resources that were tied with the file and is done using the close() method. Python has a garbage collector to clean up unreferenced objects but, we must not rely on it to close the file.
f = open(“test.txt”,encoding = ‘utf-8’)
# perform file operations
f.close()
This method is not entirely safe. If an exception occurs when we are performing some operation with the file, the code exits without closing the file. A safer way is to use a try…finally block.
try:
f = open(“test.txt”,encoding = ‘utf-8’)
# perform file operations
finally:
f.close()
This way, we are guaranteed that the file is properly closed even if an exception is raised, causing program flow to stop. The best way to do this is using the with statement. This ensures that the file is closed when the block inside with is exited. We don’t need to explicitly call the close() method. It is done internally.
with open(“test.txt”,encoding = ‘utf-8’) as f:
# perform file operations
Writing to a file
In order to write into a file we need to open it in write ‘w’, append ‘a’ or exclusive creation ‘x’ mode. We need to be careful with the ‘w’ mode as it will overwrite into the file if it already exists. All previous data are erased. Writing a string or sequence of bytes (for binary files) is done using write() method. This method returns the number of characters written to the file.with open("test.txt",'w',encoding = 'utf-8') as f: f.write("my first file\n") f.write("This file\n\n") f.write("contains three lines\n")This program will create a new file named ‘test.txt’ if it does not exist. If it does exist, it is overwritten. We must include the newline characters ourselves to distinguish different lines.
Reading from a file
To read the content of a file, we must open the file in reading mode. There are various methods available for this purpose. We can use the read(size) method to read in size number of data. If size parameter is not specified, it reads and returns up to the end of the file.
>>> f = open(“test.txt”,’r’,encoding = ‘utf-8’)
>>> f.read(4) # read the first 4 data
‘This’
>>> f.read(4) # read the next 4 data
‘ is ‘
>>> f.read() # read in the rest till end of file
‘my first file\nThis file\ncontains three lines\n’
>>> f.read() # further reading returns empty sting
”
We can see, that read() method returns newline as ‘\n’. Once the end of file is reached, we get empty string on further reading. We can change our current file cursor (position) using the seek() method. Similarly, the tell() method returns our current position (in number of bytes).
>>> f.tell() # get the current file position
56
>>> f.seek(0) # bring file cursor to initial position
0
>>> print(f.read()) # read the entire file
This is my first file
This file
contains three lines
We can read a file line-by-line using a for loop. This is both efficient and fast
>>> for line in f:
… print(line, end = ”)
…
This is my first file
This file
contains three lines
The lines in file itself has a newline character ‘\n’. Moreover,the print() end parameter to avoid two newlines when printing. Alternately, we can use readline() method to read individual lines of a file. This method reads a file till the newline, including the newline character.
>>> f.readline()
‘This is my first file\n’
>>> f.readline()
‘This file\n’
>>> f.readline()
‘contains three lines\n’
>>> f.readline()
”
Lastly, the readlines() method returns a list of remaining lines of the entire file. All these reading method return empty values when end of file (EOF) is reached.
>>> f.readlines()
[‘This is my first file\n’, ‘This file\n’, ‘contains three lines\n’]
Pythion file methods
There are various methods available with the file object. Some of them have been used in above examples. Here is the complete list of methods in text mode with a brief description.
Python File Methods
Method Description
close() Close an open file. It has no effect if the file is already closed.
detach() Separate the underlying binary buffer from the TextIOBase and return it.
fileno() Return an integer number (file descriptor) of the file.
flush() Flush the write buffer of the file stream.
isatty() Return True if the file stream is interactive.
read(n) Read atmost n characters form the file. Reads till end of file if it is negative or None.
readable() Returns True if the file stream can be read from.
readline(n=-1) Read and return one line from the file. Reads in at most n bytes if specified.
readlines(n=-1) Read and return a list of lines from the file. Reads in at most n bytes/characters if specified.
seek(offset,from=SEEK_SET) Change the file position to offset bytes, in reference to from (start, current, end).
seekable() Returns True if the file stream supports random access.
tell() Returns the current file location.
truncate(size=None) Resize the file stream to size bytes. If size is not specified, resize to current location.
writable() Returns True if the file stream can be written to.
write(s) Write string s to the file and return the number of characters written.
writelines(lines) Write a list of lines to the file.