Python File IO

CSC 223 - Advanced Scientific Programming

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