try block lets you test a block of code for errors.
except block lets you handle the error.
finally block lets you execute code, regardless of the result of the try- and except blocks.
# Print one message if the try block raises a NameError and another for other errors: try: print(x) except NameError: print("Variable x is not defined") except: print("Something else went wrong") finally: print("The 'try except' is finished")
Variable x is not defined The 'try except' is finished
List of exceptions can be found here: https://docs.python.org/3/library/exceptions.html
File handling is an important part of any web application. Python has several functions for creating, reading, updating, and deleting files.
The key function for working with files in Python is the
open() function takes two parameters; filename, and mode.
There are four different methods (modes) for opening a file:
"r" - Read - Default value. Opens a file for reading, error if the file does not exist
"a" - Append - Opens a file for appending, creates the file if it does not exist
"w" - Write - Opens a file for writing, creates the file if it does not exist
"x" - Create - Creates the specified file, returns an error if the file exists
In addition you can specify if the file should be handled as binary or text mode
"t" - Text - Default value. Text mode
"b" - Binary - Binary mode (e.g. images)
To open a file for reading it is enough to specify the name of the file:
f = open("demofile.txt")
The code above is the same as:
f = open("demofile.txt", "rt")
Because "r" for read, and "t" for text are the default values, you do not need to specify them.
By default the read() method returns the whole text, but you can also specify how many characters you want to return:
# Return the 5 first characters of the file: try: f = open("demofile.txt", "r") print(f.read(5)) except FileNotFoundError as e: print (e)
You can return one line by using the readline() method:
# Read one line of the file: try: f = open("demofile.txt", "r") print(f.readline()) except FileNotFoundError as e: print (e)
To write to an existing file, you must add a parameter to the open() function:
"a" - Append - will append to the end of the file
"w" - Write - will overwrite any existing content
# 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())
To create a new file in Python, use the
open() method, with one of the following parameters:
"x" - Create - will create a file, returns an error if the file exist
"a" - Append - will create a file if the specified file does not exist
"w" - Write - will create a file if the specified file does not exist