File

• f_obj = open(<filename>, mode='r',buffering=-1,...): a file object
  • mode:
    • r=read,
    • w=write
    • a=appending
    • x=exclusive crate(fail if already exist)
    • b=binary mode
    • +=open a disk file for updating(read+write, eg. r+ or a+)
    • t=text mode(default)
  • buffering: 0=off, 1=line buffering, >1 indicate the size in bytes a fixed-size chunk-buffer
  • iterative
  • methods
    • .read([size]), .readline(), .readlines()
    • .write(), .writelines()
    • .close()
    • .seek(offset, whence=0): with offset bytes from whence(0=begin,1=current,2=end)
      • when falls in the middle of unicode could cause error
    • truncate() - erase the content of file

Context Manager

with open(<path>, [mode]) as <var>: with block

• automatic clean-up (no need to close)

• standard file: sys.stdin/stdout/stderr

• text mode

  • the default mode - work with file as python strings(i.e. Unicode)
  • UTF-8: may need to decode to str yourself
    • data.decode('utf8')
  • with open(..., encoding = 'iso-8859') as f:
    • from one unicode to the other unicode

with works with objects that have __enter__ and __exit__ methods (eg. the output of open())

try:
    f = open('/path/to/file', 'r')
    f.read()
finally:
    if f:
        f.close()

# is equivalent to
with open('/path/to/file', 'r') as f:
    f.read()

# context object
class Query(object):

    def __init__(self, name):
        self.name = name

    def __enter__(self):
        print('Begin')
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type:
            print('Error')
        else:
            print('End')

    def query(self):
        print('Query info about %s...' % self.name)

with Query('Bob') as q:
    q.query()