Kanel Nang

 Two methods of formatting output ◦ Standard string slicing and concatenation operations ◦ str.format() method ie. >>> a = “The sum of is {0}”.format(1+2) >>> print a The sum of is 3

 How do you convert value to string in Python? ◦ Functions  Repr()  Str() ◦ Differences  Str() function is meant to return representation of values that is human-readable  Repr() generate representations can be read by interpreter  If str() does have a proper human representation, then it will return the same value as repr()

 >>> s = ‘hello, world\n’  >>> hello = repr(s)  >>> print hello  ‘hello, world\n’  >>> hellos = str(s)  >>> print hellos  hello, world  >>> x = 10 * 3.25  >>> y = 200 * 200  >>> s = ‘The value of x is ‘ + repr(x) + ‘, and y is ‘ + repr(y) + ‘…’  >>> print s  The value of x is 32.5, and y is 40000…

 Useful methods of string objects ◦ rjust()  Right-justifies a string in a field of a given width by padding spaces on the left ◦ ljust()  Left-justifies a string in a field ◦ center()  Center-justifies a string in a field ◦ zfill() Pads a numeric string on the left with zeros

 >>> for x in range(1, 11): ...print repr(x).rjust(2), repr(x*x).rjust(3), ...# Note trailing comma on previous line ...print repr(x*x*x).rjust(4)          

 >>> ‘12’.zfill(5)  ‘00012’  >>> ‘-3.14’.zfill(7)  ‘ ’  >>> ’ ’.zfill(5)  ‘ ’

 Old string formatting ◦ The % operator is another method for string formatting. ◦ Similar to sprintf() in that it interprets the left argument to be applied to the right argument, then return the result of the string.  ie. >>> import math >>> print ‘The value of PI is approximately %5.3f.’ % math.pi The value of PI is approximately 3.142

 Open() ◦ Returns a file object and is most commonly used with two arguments  ie.  >>> open(filename, mode)  Modes ◦ ‘r’ – read only ◦ ‘w’ - writing only (overwrite existing files with same name) ◦ ‘a’ – opens file for appending (new data added to the end) ◦ ‘r+’ - opens the file in both reading and writing ◦ The mode argument is optional, ‘r’ will be assumed if it’s omitted

 Other modes ◦ On Windows  ‘b’ appended to the mode opens file in binary mode  ie.  ‘rb’, ‘wb’, and ‘r+b’ ◦ Python on windows will make a distinction between text and binary files, therefore the end-of-line characters in text files will be automatically altered slighted when data is read or written  Safe for ASCII text files, currupt binary data such as JPEG or EXE files

 Methods ◦.read(size) - reads some quantity of data and returns it as a string, if size is omitted, the entire content will be read and returned ◦.readline() - reads a single line from a file (\n should be left at the end of a string and should only be omitted on the last line ◦.readlines() - returns a list containing all the lines of a data file (size bytes can be included to avoid overloading memory) ◦.write() – writes the content of a string to the file ◦.tell() – returns an integer giving the file object’s current position in the file measured in bytes ◦.seek() - change the files object’s position ◦.close() – closes the file

 >>> F.read()  ‘This is the entre file.\n’  >>> F.readline()  ‘This is the firstline of the file.\n’  >>> F.readline()  ‘Second line of the file\n’  >>> F.readlines()  [‘This is the first line of the file.\n’, ‘Second line of the file\n’]  >>> F = open(‘/tmp/workfile’, ‘r+’)  >>> F.write(‘ abcdef’)  >>> F.seek(5) #Go to the 6 th byte in the file  >>> F.read(1)  ‘5’

 pickling ◦ A module that can take almost any Python object and convert it to a string representation.  ie.  You have object x and a file object f that’s open for writing  Pickle.dump(x, f)  unpickling ◦ Reconstructing the object from the string representation ◦ ie. ◦ f is a file object opened for reading ◦ X = pickle.load(f) The standard way to make Python objects which can be stored and reused by other programs

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