Python File IO

File IO

• The open function returns a file object.

• Basic syntax
  open(filename, mode)

• The mode parameter is a string that specifies permissions:

  • ’r’ – read

  • ’w’ – write (existing file with same name is erased)

  • ’a’ – append

  • ’r+’ – read and write

Basic Methods on File Objects

• read – read the contents of a file

• readline – read a single line from the file

• write – write to a file

• close – close the file

• File objects can be treated as iterators

File Example

# copy the contents of one file to another
inFile = open('myfile_in')
outFile = open('myfile_out', 'w')

for line in file:
    outFile.write(line)

inFile.close()
outFile.close()

The with Keyword

• The with keyword can be used for a category of objects called context managers.

• A context manager has implementations of the __enter__ and __exit__ methods.

• The with statement guarantees that the __exit__ method is called at the end of the block.

• The __exit__ method of a file object closes the file.

• Example:

  with open('myfile.txt', 'w') as f:
      f.write('Hello world!')
  # the file is guaranteed to be closed here

File Formats

• Python has many modules to handle many file formats commonly used in data science tasks:

  • Comma separated values (CSV)

  • Hypertext Markup Language (HTML)

  • Extensible Markup Language (XML)

  • JavaScript Object Notation (JSON)

  • Sqlite database files