Files , files have been something we are dealing with every so ofen, as an example, we might have come across such code:

file = open('examples/rules.txt', encoding='utf-8')

two things to notice for this concise code:

1. use forward slash to denote subdirectories even on Windows
2. encoding is used to indicate the proper decoding system, because the default one is platform-dependent

So, what does the open() function return? It is a stream object, which has methods and attributes for getting information about and manipulating a stream of characters, like .name, .encoding, .mode, by default, the mode is read.

>>> a_file = open('examples/chinese.txt', encoding='utf-8')
>>> a_file.read()
'Dive Into Python 是为有经验的程序员编写的一本 Python 书。\n'
>>> a_file.seek(17)
17
>>> a_file.read(1)
'是'
>>> a_file.tell()
20

seek method moves to a specific byte position, read takes the number of characters to read, so when you tell the byte position now, it has moved another 3 position rather than one. And since Chinese characters requires multiple bytes to encode, you should be aware that you cannot read from the middle of a set of constituting bytes of one character.

finally close it by using .close(), the stream object then still exists with only a closed attribute. But the more preferred manner should be using with statement:

with open('examples/chinese.txt', encoding='utf-8') as a_file:

It’s more secure to use with code block, it guarantees that your file will be closed however your program halts. That’s because the with statement creates a so-called runtime context, and the stream object calls its own close() method once Python exits that context.

The stream object is an iterator itself, you can use a for loop to read one line at a time:

line_number = 0
with open('name.txt', encoding='utf-8') as a_file:
    for a_line in a_file:
        line_number += 1
        print('{:>4} {}'.format(line_number, a_line.rstrip()))

just in case it’s unfamiliar: {:>4} means “print this argument right-justified within 4 spaces.

*of course, you can use the method readline() as well as readlines()

reading and appending is achieved through the mode parameter ‘w’ and ‘a’.

Read things beyond text files

What if we want to read something like a picture? The same old tricks shall work, except that the mode parameter should now be rb, wb, or ab;

But we can even read things beyond files. By using StringIo class in io module, we create an instance of theclass and pass it the string you want to use as your “file” data.

>>> a_string = 'PapayaWhip is the new black.'
>>> import io
>>> a_file = io.StringIO(a_string)

The stream object you now have get all those methods and attributes as well.

Standard Input, output and error

Standard output and standard error (commonly abbreviated stdout and stderr) are pipes that are built into every U N I X -like system, including Mac OS X and Linux. When you call the print() function, the thing you’re printing is sent to the stdout pipe. When your program crashes and prints out a traceback, it goes to the stderr pipe.

If that is understood, which is to say the nature of print() is adding a carriage return and calling sys.stdout.write¹, we can direct a different output place, with a custom context manager.

A class is a context-manager as long as it has __enter__() and __exit__() methods.

class RedirectStdoutTo:
    def __init__(self,out_new):
        self.out_new = out_new

    def __enter__(self):
        self.out_old = sys.stdout
        sys.stdout = self.out_new

    def __exit__(self, exc_type, exc_val, exc_tb):
        sys.stdout = self.out_old

Then we can use with statement to automatically call __enter__() and __exit__(). A bit tricky though, is that we can use with statement to take a comma-separated list of contexts.

with open('out.log', mode='w', encoding='utf-8') as a_file, RedirectStdoutTo(a_file):
print('B')

The first context listed is the “outer” block; the last one listed is the “inner” block. Rewrite it would be like

with open('out.log', mode='w', encoding='utf-8') as a_file:
    with RedirectStdoutTo(a_file):
        print('B')