Fancier Output Formatting
So far we’ve encountered two ways of writing values: expression statements and the print statement. (A third way is using the write() method of file objects; the standard output file can be referenced as sys.stdoutLuckily, Python has ways to convert any value to a string: pass it to the repr() or str() functions.
The str() function is meant to return representations of values which are fairly human-readable, while repr() is meant to generate representations which can be read by the interpreter (or will force a SyntaxError if there is not equivalent syntax). For objects which don’t have a particular representation for human consumption, str() will return the same value as repr(). Many values, such as numbers or structures like lists and dictionaries, have the same representation using either function. Strings and floating point numbers, in particular, have two distinct representations.
Some examples:
>>> s = 'Hello, world.' >>> str(s) 'Hello, world.' >>> repr(s) "'Hello, world.'" >>> str(1.0/7.0) '0.142857142857' >>> repr(1.0/7.0) '0.14285714285714285'
Basic usage of the str.format() method looks like this:
>>> print 'We are the {} who say "{}!"'.format('knights', 'Ni')
We are the knights who say "Ni!"The brackets and characters within them (called format fields) are replaced with the objects passed into the format() method. A number in the brackets refers to the position of the object passed into the format() method.>>> print '{0} and {1}'.format('spam', 'eggs')
spam and eggs
>>> print '{1} and {0}'.format('spam', 'eggs')
eggs and spamPassing an integer after the ':' will cause that field to be a minimum number of characters wide. This is useful for making tables pretty.>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
... print '{0:10} ==> {1:10d}'.format(name, phone)
...
Jack ==> 4098
Dcab ==> 7678
Sjoerd ==> 4127If you have a really long format string that you don’t want to split up, it would be nice if you could reference the variables to be formatted by name instead of by position. This can be done by simply passing the dict and using square brackets '[]' to access the keys>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print ('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '
... 'Dcab: {0[Dcab]:d}'.format(table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678Old string formatting
The % operator can also be used for string formatting. It interprets the left argument much like a sprintf()-style format string to be applied to the right argument, and returns the string resulting from this formatting operation. For example:>>> import math >>> print 'The value of PI is approximately %5.3f.' % math.pi The value of PI is approximately 3.142.
Reading and Writing Files
open() returns a file object, and is most commonly used with two arguments: open(filename, mode).>>> f = open('/tmp/workfile', 'w')
>>> print f
<open file '/tmp/workfile', mode 'w' at 80a0960>The first argument is a string containing the filename. The second argument is another string containing a few characters describing the way in which the file will be used. mode can be 'r' when the file will only be read, 'w' for only writing (an existing file with the same name will be erased), and 'a' opens the file for appending; any data written to the file is automatically added to the end. 'r+' opens the file for both reading and writing. The mode argument is optional; 'r' will be assumed if it’s omitted.Methods of File Objects
The rest of the examples in this section will assume that a file object called f has already been created.To read a file’s contents, call f.read(size), which reads some quantity of data and returns it as a string. size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned; it’s your problem if the file is twice as large as your machine’s memory. Otherwise, at most size bytes are read and returned. If the end of the file has been reached, f.read() will return an empty string ("").
>>> f.read() 'This is the entire file.\n' >>> f.read() ''
f.readline() reads a single line from the file; a newline character (\n) is left at the end of the string, and is only omitted on the last line of the file if the file doesn’t end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by '\n', a string containing only a single newline.
>>> f.readline() 'This is the first line of the file.\n' >>> f.readline() 'Second line of the file\n' >>> f.readline() ''f.readlines() returns a list containing all the lines of data in the file. If given an optional parameter sizehint, it reads that many bytes from the file and enough more to complete a line, and returns the lines from that. This is often used to allow efficient reading of a large file by lines, but without having to load the entire file in memory. Only complete lines will be returned.
>>> f.readlines() ['This is the first line of the file.\n', 'Second line of the file\n']
f.write(string) writes the contents of string to the file, returning None.
>>> f.write('This is a test\n')To write something other than a string, it needs to be converted to a string first:>>> value = ('the answer', 42)
>>> s = str(value)
>>> f.write(s)f.tell() returns an integer giving the file object’s current position in the file, measured in bytes from the beginning of the file. To change the file object’s position, use f.seek(offset, from_what). The position is computed from adding offset to a reference point; the reference point is selected by the from_what argument. A from_what value of 0 measures from the beginning of the file, 1 uses the current file position, and 2 uses the end of the file as the reference point. from_what can be omitted and defaults to 0, using the beginning of the file as the reference point.
>>> f = open('/tmp/workfile', 'r+')
>>> f.write('0123456789abcdef')
>>> f.seek(5) # Go to the 6th byte in the file
>>> f.read(1)
'5'
>>> f.seek(-3, 2) # Go to the 3rd byte before the end
>>> f.read(1)
'd'When you’re done with a file, call f.close() to close it and free up any system resources taken up by the open file. After calling f.close(), attempts to use the file object will automatically fail.
>>> f.close() >>> f.read() Traceback (most recent call last): File "<stdin>", line 1, in ? ValueError: I/O operation on closed file
It is good practice to use the “ with “ keyword when dealing with file objects. This has the advantage that the file is properly closed after its suite finishes, even if an exception is raised on the way. It is also much shorter than writing equivalent try-finally blocks:
>>> with open('/tmp/workfile', 'r') as f:
... read_data = f.read()
>>> f.closedThe pickle Module
Strings can easily be written to and read from a file. Numbers take a bit more effort, since the read() method only returns strings, which will have to be passed to a function like int(), which takes a string like '123' and returns its numeric value 123. However, when you want to save more complex data types like lists, dictionaries, or class instances, things get a lot more complicated.Rather than have users be constantly writing and debugging code to save complicated data types, Python provides a standard module called pickle. This is an amazing module that can take almost any Python object (even some forms of Python code!), and convert it to a string representation; this process is called pickling. Reconstructing the object from the string representation is called unpickling. Between pickling and unpickling, the string representing the object may have been stored in a file or data, or sent over a network connection to some distant machine.
If you have an object x, and a file object f that’s been opened for writing, the simplest way to pickle the object takes only one line of code:
pickle.dump(x, f)To unpickle the object again, if f is a file object which has been opened for reading:
x = pickle.load(f)
gud keep it up
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