Reading Text Files

• File Objects
• Context manager
• Read method
• Readlines Method
• File object iterator

Syntax

fileopbj = open(filename, mode)

• fileopbj is the fileopbject returned by open()
• filename is the string name of the file
• mode is a string indicating the file’s type and what you want to do with it

mode	operation
‘r’	read
‘w’	write
‘a’	append
‘x’	write but only if it doesn exists
‘t’	text
‘b’	binary

• “x” - Create - will create a file, returns an error if the file exist
• ‘w’ - Create a new file if it does not exist:

File Objects

• provide acces to a file
• have read and write methods
• have a filepointer
  • is like an indiex, it tells us position in the data while reading or writing

open() function

You need to cal the open() before you:

• read an existing file
• write a new file
• append to an exiting file
• overwrite an exiting file

Syntax

open(filename, mode='r')

• return a file object from filename
• made sets the file type and either read or write mode
• default mode is read mode

operation	mode
‘r’	read
‘w’	write
‘a’	append

‘a’ “a” - Append - will append to the end of the file “w” - Write - will overwrite any existing content

Write a Textfile with print()

fout = open(oops.txt, 'wt')
print('Oops, I created a file.', file=fout)
fout.close()

Using file argument to print

without the file argument to print, print just writes to standard output > terminal. [unless you have told the shell program to redirect output to a file with > or piped it to another program with | ]

Write a Textfile with write()

poem = '''There was a yong lady named Bright, 
Whose pseed was faster then light;'''

fout = open('speed', 'wt')
fout.write(poem)
# Output
fout.close()

Write() returns the number od bytes written. It does not add any spaces or newlines, as print does. To make print() work like write(), you pass the arguments sep - called seperator (‘ ‘) and end called end string (\n)

• you can print a multiline string to a text file

poem = '''There was a yong lady named Bright, 
Whose pseed was faster then light;'''

fout = open('speed', 'wt')
print(poem, file=fout)
# print(poem, file=fout sep'', end='')
fout.close()

try:
  fout = open('relativity', 'xt')
  fout.write('stomp, stromp, stomp')
excpt FileExistsError:
  print("relativity already exists. overriding prevented")

Getting a File Object (read mode)

f = open(filename, mode='r')
file_access_codeblock
f.close()

closing the file object releases resources

Context manager

• prefered way to work with file objects
• idiomatic way to access files
• it does the same thing but it makes the code clear.
• Automatically closes file object

Syntax

with open(filename, 'r') as f:
    file_access_codeblock

• with keyword indicates the start of a contect manager
• open() function
• as keyword think of it as a assignment operator but reversed

Read method

Syntax

f.read()

Example1

f = open("demofile.txt", "r")
print(f.read())

Example2

fin = open("relativity", "rt")
poem = fin.read()
fin.close()

• slurp up the entire file at once
• read from f and return its entire contents as a string

with open('lines.txt', 'r') as f: 
    contents = f.read()
    print(contents)

• The read() method returns the specified number of bytes from the file. Default is -1 which means the whole file.
• Now read 100 characters at a time and append each chunk to a poem string

poem = ''
fin = open("relativity", "rt")
chunk = 100
while True: 
  fragment = fin.read(chunk)
  if not fragment: 
    break
  poem += fragment
fin.close()
len(poem)
# Output
# # 150)

Readlines Method

• reading a line at a time

Syntax

f.readlines()

• read from f and return its entire contents as a list of strings
• each string is delimited by a newline(inclusive)
• file pointer moves to the end of the file

with open('lines.txt', 'r') as f: 
    lines = f.readlines()
    print(lines)
    # Output
    # ['On your marks.\n', 'Get set.\n'.'Go!\n']

Write a Test File with write()

• just like print

File object iterator

• Files objects are also iterators
  • file is read line by line, and not all at once
  • at each iteration a string is returned
  • String contents start from the file pointer up to and including a newline
  • filepointer is placed after the end of the new line ready for the next read operation.

File object iterator Example

# using the file object as an iterator
with open('lines.txt', 'r') as f: 
    for line in f:
        # we iterate though the file object using a for loop
        # each iteration the variable line will be set to a line from lines.txt
        print(line, end='')
        # file pointer has moved to a next line.

=> page 263

From the book [O’Reilly, Introducing Python]

flat files

• read form a file into memory and write from memory to the file.

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	Strings		Variables		Lists		Tuples		Dictionary
	Control		Function		Files		Exceptions
	OOP		Algorithm		Data Structure		back

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