Python - File Handling,Exceptional Handling and Debugging.

File Handling

• File is a named location on disk to store related information.

• Whenever we read/write , first we need to open a file .

• Once done , we should close the file so that resources tied to it are released.

• Mainly 5 file operations :

1. Opening a file.

2. Closing a file.

3. Writing to a File.

4. Reading from a file .

5. File and OS commands .

1. Opening a file :

eg :

>>> f = open('example.txt') ## opens a file in current directory , f is a file handler/pointer.

Modes :

'r' - Opens for reading (default).

'w'- Writing - Creates a new file if it does not exists or truncates if file already exists.

'x' - Opens a file for exclusive creation ie if file exits then operation will fail.

'a' - Opens for appending at end of file, creates a new file if it does not exists.

't' - Opens in text mode (default).

'b' - Opens a file in binary mode.

'+' - Opens a file for updating (reading and writing).

- Default encoding (Python interpreter cannot talk to files directly ie it goes to os which gets file - utf-8 on linux and returns back to python interpreter ) is platform dependent ie windows - 'cp1252' and linux 'utf-8'.

- Always working with text files it is recommended to specify encoding type.

>>> f=open ('example.txt','r',encoding='utf-8')

2. Closing a file :

• Frees up resources that were tied with the file.

• Close is used so that file can move from RAM to HDD.

• Python has garbage collector to clean up unreferenced objects but , we must not rely on it to close the file.

• If there is any exception then file may not get closed , so preferred to use with finally (always a cleaner and safer way ).

• We can close without close () function .

eg :

3. Writing to a file :

• In order to write to a file we must first open it in write 'w' or append 'a' mode or exclusive creation 'x' mode.

• Writing a string or sequence of bytes (for binary files) is done using write() method.This method returns number of characters written to a file.

• eg :

4. Reading from a file :

eg :

5. File and OS commands :

• Renaming and Deleting files :

eg :

• Directory and File Management : eg:

eg :

Exceptional Handling

• Errors caused by not following proper structure (syntax) of the language are called syntax error or parsing error.

• By default as soon as error is raised by python parser program execution stopes there.

• eg :

• Errors can also occur at runtime and these are called as exceptions .

• eg :

• All types of exceptions which python handles can be viewed by dir(__builtins__)

• eg:

• Try ,Except and Finally :

- If no error then except block is not executed.

eg :

- We can forcefully raise exceptions .

eg :

- No matter what is executed finally always gets executed, generally used to release resources.

eg :

Debugging

• pdb(python debugger) implements an interactive debugging environment for python programs .

• Starting debugger :

eg:

• I/p values :

- c : Continue.

- q : Quit .

- h: Help.

- list : Lists where exactly we are.

- p: Print

- p locals() : Prints local variables value.

- p globals() : Prints all global variables .

- h: Here - Prints a stack trace.

- d : Down[count] - moves the current count (default 1) level down in stack trace.