Data File Handling

try:
    f = open("myfile.txt", "r")
    f.close()
except IOError:
    print "File does not exist."

# Example: write()
f = open("out1.txt", "w")
f.write("Welcome to file handling\n")
f.close()

# Example: writelines()
g = open("out2.txt", "w")
g.writelines(["Line 1\n", "Line 2\n", "Line 3\n"])
g.close()

import pickle

# Write a structured record to binary file, read it back, and display it
def write_read_structure():
    # Example structure (dictionary)
    student = {"roll": 101, "name": "Riya", "marks": [90, 88, 95]}

    # Store structure in binary file
    f = open("student.dat", "wb")
    pickle.dump(student, f)
    f.close()

    # Read same structure back from file
    f = open("student.dat", "rb")
    data = pickle.load(f)
    f.close()

    print data

# Read textfile.txt and print words with serial numbers
def display_numbered_words():
    f = open("textfile.txt", "r")
    i = 1
    for line in f:
        # strip removes newline before display
        print i, ":", line.strip()
        i = i + 1
    f.close()

# Take words from user and stop when END is entered
f = open("textfile.txt", "w")
while True:
    word = raw_input("Enter a word (END to quit): ")
    # Do not write END into the file
    if word.upper() == "END":
        break
    f.write(word + "\n")
f.close()

# Apply a transformation function to each input line
def process_file(fin, fout, transform):
    for line in fin:
        fout.write(transform(line))

# Sample transformation: capitalize each word
def capitalize_words(s):
    return s.title()

# Open files and process line by line
infile = open("input.txt", "r")
outfile = open("output.txt", "w")
process_file(infile, outfile, capitalize_words)
infile.close()
outfile.close()

python is fun
file handling is important
cbse class twelve

Python Is Fun
File Handling Is Important
Cbse Class Twelve

# Perform all file operations asked in Q14
def q14_solution():
    f = open("funny.txt", "w")
    # Write each sentence on a separate line
    f.write("Neither apple nor pine are in pineapple.\n")
    f.write("Boxing rings are square.\n")
    f.write("Writers write, but fingers don't fing.\n")
    f.write("Overlook and oversee are opposites.\n")
    f.write("A house can burn up as it burns down.\n")
    f.write("An alarm goes off by going on.\n")
    f.close()

    # (i)
    f = open("funny.txt", "r")
    print f.read()
    f.close()

    # (ii)
    f = open("funny.txt", "a")
    f.write("Practice makes a person better.\n")
    f.close()

    f = open("funny.txt", "r")
    lines = f.readlines()
    f.close()

    i = 1
    for line in lines:
        print i, ":", line.rstrip()
        i = i + 1

    # (iii)
    print "Last line:", lines[-1].rstrip()

    # (iv)
    print "First line from 10th character:", lines[0][9:].rstrip()

    # (v)
    ln = int(raw_input("Enter line number: "))
    if ln >= 1 and ln <= len(lines):
        print lines[ln - 1].rstrip()
    else:
        print "Invalid line number"

import pickle

# Store Roman numeral dictionary in a binary file
def create_roman_dict_file():
    roman = {
        'I': 1,
        'V': 5,
        'X': 10,
        'L': 50,
        'C': 100,
        'D': 500,
        'M': 1000
    }
    # Open file in binary write mode and store dictionary
    f = open("roman.dat", "wb")
    pickle.dump(roman, f)
    f.close()

# Convert Roman string to decimal using binary-file dictionary
def roman_to_decimal(num):
    # Load dictionary back from binary file
    f = open("roman.dat", "rb")
    d = pickle.load(f)
    f.close()

    total = 0
    i = 0
    while i < len(num):
        # Get current symbol value
        val = d[num[i]]
        if i + 1 < len(num) and val < d[num[i + 1]]:
            # Subtractive case like IV, IX, XL, etc.
            total = total + (d[num[i + 1]] - val)
            i = i + 2
        else:
            total = total + val
            i = i + 1
    return total

{'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000}

roman_to_decimal("MCMIV") -> 1904

# Stream through binary file and copy only required records
import pickle
import os

def delete_content(fname, target):
    # Read original file and write filtered records to temp file
    fin = open(fname, "rb")
    fout = open("temp.dat", "wb")

    try:
        while True:
            # Read one record at a time (memory efficient)
            rec = pickle.load(fin)
            # Skip only the target record
            if rec != target:
                pickle.dump(rec, fout)
    except EOFError:
        # End of file reached
        pass

    # Replace original file with updated temp file
    fin.close()
    fout.close()
    os.remove(fname)
    os.rename("temp.dat", fname)

[10, 25, 30, 25, 40]

[10, 30, 40]

import os

# Insert a sentence at a specific line using a temporary file
def insert_sentence_in_file(fname, sentence, line_no):
    # Open original for read and temp for write
    fin = open(fname, "r")
    fout = open("temp.txt", "w")

    i = 1
    inserted = False
    for line in fin:
        # Insert sentence before requested line number
        if i == line_no:
            fout.write(sentence + "\n")
            inserted = True
        fout.write(line)
        i = i + 1

    if not inserted:
        # If line_no is beyond file length, append at end
        fout.write(sentence + "\n")

    fin.close()
    fout.close()

    # Replace original file with modified file
    os.remove(fname)
    os.rename("temp.txt", fname)

Line 1: Python
Line 2: File Handling
Line 3: Binary File

Line 1: Python
Inserted Line
Line 2: File Handling
Line 3: Binary File

# Build word index: word -> list of line numbers
def build_index(infile, outfile):
    # Dictionary to store word occurrences
    index = {}
    f = open(infile, "r")

    line_no = 1
    for line in f:
        # Normalize and split into words
        words = line.lower().replace(",", " ").replace(".", " ").split()
        for w in words:
            # Add first occurrence or append new line number
            if w not in index:
                index[w] = [line_no]
            elif index[w][-1] != line_no:
                index[w].append(line_no)
        line_no = line_no + 1

    f.close()

    # Write index in sorted word order
    out = open(outfile, "w")
    for word in sorted(index):
        out.write(word + " : " + str(index[word]) + "\n")
    out.close()

Python is powerful.
Python is easy to learn.
Files are useful in Python.

are : [3]
easy : [2]
files : [3]
in : [3]
is : [1, 2]
learn : [2]
powerful : [1]
python : [1, 2, 3]
to : [2]
useful : [3]

# Copy file1 to file2 while replacing pattern with replacement
def replace_file(pattern, replacement, file1, file2):
    # Open source and destination files
    f1 = open(file1, "r")
    f2 = open(file2, "w")

    for line in f1:
        # Replace all occurrences in each line
        f2.write(line.replace(pattern, replacement))

    f1.close()
    f2.close()

Python is simple.
I like Python file handling.

CBSE Python is simple.
I like CBSE Python file handling.

# Print only those lines that contain '#'
fname = raw_input("Enter filename: ")
# Open file in read mode
f = open(fname, "r")

for line in f:
    if "#" in line:
        print line.rstrip()

f.close()

print "Hello"
# This is a comment
x = 10  # inline comment
print x

# This is a comment
x = 10  # inline comment

# Read file lines and display them in reverse order
fname = raw_input("Enter filename: ")
# Read all lines first
f = open(fname, "r")
lines = f.readlines()
f.close()

# Print from last line to first line
for i in range(len(lines) - 1, -1, -1):
    print lines[i].rstrip()

Start program
Read data
Write report
Program end

Program end
Write report
Read data
Start program

import pickle
import os

# Class to store item details
class Item(object):
    def __init__(self, code, name, price, qty):
        self.code = code
        self.name = name
        self.price = price
        self.qty = qty

# Add multiple item records to binary file
def add_items(fname):
    # Open file in append-binary mode to keep old records
    f = open(fname, "ab")
    n = int(raw_input("How many items? "))
    for i in range(n):
        c = raw_input("Code: ")
        nm = raw_input("Name: ")
        p = float(raw_input("Price: "))
        q = int(raw_input("Qty: "))
        pickle.dump(Item(c, nm, p, q), f)
    f.close()

# Search an item by code and display details
def search_item(fname, code):
    # Read records one-by-one till match or EOF
    f = open(fname, "rb")
    found = False
    try:
        while True:
            rec = pickle.load(f)
            if rec.code == code:
                print rec.code, rec.name, rec.price, rec.qty
                found = True
                break
    except EOFError:
        # No more records
        pass
    f.close()
    if not found:
        print "Item not found"

# Update quantity of an item using temporary file technique
def update_item_qty(fname, code, new_qty):
    # Copy all records to temp; modify matching record
    fin = open(fname, "rb")
    fout = open("temp_item.dat", "wb")

    try:
        while True:
            rec = pickle.load(fin)
            if rec.code == code:
                rec.qty = new_qty
            pickle.dump(rec, fout)
    except EOFError:
        # End of records
        pass

    # Replace old file with updated file
    fin.close()
    fout.close()
    os.remove(fname)
    os.rename("temp_item.dat", fname)

[('I101', 'Pen', 12.5, 50),
 ('I102', 'Notebook', 45.0, 30),
 ('I103', 'Pencil', 5.0, 80)]

I102 Notebook 45.0 30

[('I101', 'Pen', 12.5, 50),
 ('I102', 'Notebook', 45.0, 45),
 ('I103', 'Pencil', 5.0, 80)]