Fundamentals of Programming(Python)

File Processing

Ali TaheriSharif University of Technology

Fall 2018

Outline1. Sources of Input

2. Files

3. Opening a File

4. Opening Modes

5. Closing a File

6. Writing to a File

7. Reading from a File

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Sources of InputUser Input◦ Slow if needed to enter a lot of data

◦ Error prone: user may enter wrong values

◦ What if we want to run again?

◦ What if we want an input based on inputs of previous executions.

Files◦ Enter data once into a file, save it, and reuse it.

◦ Good for large amounts of data

◦ Programs can use files to communicate

◦ Need to be able to read from and write to files

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FilesIn Python, a file operation takes place in the following order:1. Open the file

2. Read or write (perform operation)

3. Close the file

Files can be opened for different purposes◦ Reading, writing, updating

Open files must be closed◦ To release the resources

◦ To flush the output

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Opening a FileTo access a file, it must first be opened

The open function is used to open a file◦ <variable> = open(<path>, <mode>)◦ <variable> is of type file (file is a data type!)

◦ <path> is the path and name of the file

◦ <mode> is the opening mode (read, write, update)

Example:

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f = open('output.txt', 'w')

f = open('C:/Python36/README.txt', 'r')

Opening Modes

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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.

‘a’Open for appending at the end of the file without truncating it. Creates a new file if it does not exist.

‘b’ Open in binary mode.

‘+’ Open a file for updating (reading and writing)

f = open("test.txt") # equivalent to 'r'

f = open("test.txt",'r+') # read from begin, write at end

f = open("img.bmp",'wb') # write in binary mode

Closing a FileWhen we are done with operations to the file, we need to properly close the file

The close method is used to close the file

It can be automatically done using with statement

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f = open('output.txt', 'w')

# some operations here

f.close()

with open('data.txt', 'a+') as f:

# some operations here

# now the file has been closed

Writing to a FileThe write method is used to output data to a file

◦ Slow if needed to enter a lot of data

◦ Error prone: user may enter wrong values

◦ We must include the newline characters ourselves to distinguish different lines.

This can be done using the print function as well

◦ The print function adds a new line to the end of the string by default

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with open('data.txt', 'w') as f:

f.write("my first file\n")

f.write("This file\n\n")

f.write("contains four lines\n")

print('Sample text', file=f)

Reading from a FileWhen a file is opened for reading, an input pointer is positioned at the beginning of the file

After reading, the pointer is moved after the last read character

Reading in the text mode returns data in string

When the input pointer reaches the end of file, any further read attempts results in an empty string

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Reading from a FileReading Characters◦ The read method is used to read characters from a

file

Reading Lines◦ The readline method is used to read a single line of text

◦ The lineReturns empty string when reaches end of file

◦ contains ending newline character ‘\n’

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with open('input.txt', 'r') as f:

s = f.read(10) # read 10 characters

s = f.read() # read to the end of file

S = f.readline() # read the next line

Reading from a FileReading Lines◦ The readlines method returns all the lines as a list

◦ All the lines contain ending newline character ‘\n’

◦ We can also iterate over lines using a for loop

◦ Each line contain ending newline character ‘\n’

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with open('input.txt', 'r') as f:

lines = f.readlines() # returns all lines as a list

for line in f:

print(line, end='')

Binary FilesWriting to a file◦ We can use pickle module to operations on binary files

◦ Using dump function, we can write a variable to a file

Reading from a file◦ We can use load function to read the variable in a file

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import pickle

var = [(1, 'x', [3,4]), {1,2,3}]

with open(‘file.txt', ‘wb') as f:

pickle.dump(var, f) # Writes “var” to “file.txt”

with open(‘file.txt', ‘rb') as f:

print(pickle.load(f)) # Reads the variable in “file.txt”