Python: Input and Output

1. Python: Input and Output Yuen Kwan Lo

2. Output Format str( ) and repr( ) same representation but String and Floating point number a=0.24 str(a) ‘0.24’ repr (a) ‘0.2399999999999999999’ s = 'Hello, world.' str(s) 'Hello, world.' repr(s) "'Hello, world.'" print ( repr(s) ) ‘Hello, word.’

3. rjust() , ljust(), center() • Returns string justified in a string of given length str = “hello world” print str.rjust (20) >> hello world print str.rjust(20, ‘!’) >> !!!!!!!!!hello world zfill() • Pads a numeric string on the left with zeros '-3.14'.zfill(7) '-003.14'

4. for x in range(1, 5): print repr(x).rjust(2), repr(x*x).rjust(3), print repr(x*x*x).rjust(4) format() for x in range(1, 5): print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x) print ‘PI : {0:.3f}'.format(math.pi) >> PI: 3.142 • !s ( str() ) and !r ( repr() ) print ‘PI : {!r}'.format(math.pi) >> PI: 3.141592653589793 print ‘PI : {!s}'.format(math.pi) >> PI: 3.14159265359

5. Print set of variables print '{1} and {0}'.format('spam', 'eggs') >> eggs and spam print '{dollar} dollars and {cent} cents'.format(dollar=‘ten’, cent=‘forty’) >> ten dollars and forty cents • Print table table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678} for name, phone in table.items(): print '{0:10} ==> {1:10d}'.format(name, phone) table = {'Sjoerd': 4127, 'Dcab': 8637678} print (‘Sjoerd: {0[Sjoerd]:d}; Dcab: {0[Dcab]:d}'.format(table)) >> Jack: 4098; Sjoerd: 4127; Dcab: 8637678 print 'Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)

6. String Formatting veg = “apple” color = “red” print veg + “ is “ + color >> apple is red print “%s is %s” % (veg, color) >> apple is red count= 6 print “There are %d %s” % (count, veg) >> There are 6 apple print “There are “ + count + veg ERROR!! • Numbers Formatting print "Today's stock price: %f" % 50.4625 >> Today's stock price: 50.462500 print "Today's stock price: %.2f" % 50.4625 >> Today's stock price: 50.46 print "Change since yesterday: %+.2f" % 1.5 >> Change since yesterday: +1.50

7. Reading and Writing Files open() • open(filename, mode) f = open('/cs/PA', 'w') mode: ‘r’ read ‘w’ write *file with same name will be erased* ‘a’ append ‘r+’ read and write ‘b’ open file in binary mode (on Windows)

8. Methods of File Objects • f.read(size) f.read() **read the entire file and return string** f.read(1) **read one character at a time** • f.readline() reads a single line from the file until it reaches ‘\n’ • f.readlines() Read all lines and returns a list of all lines >>['This is the first line of the file.\n', 'Second line of the file\n']

9. f.write(string) write the contents of string to the file f.write(‘first line\n’) **make numbers to string before writing** s = str (0.24) f.write(s) • f.tell() return current position in file measured in bytes from the beginning of the file • f.close() close file and file object cannot be used unless being reopened

10. f.seek(offset, from) from: 0: beginning of file 1: current position 2: end of file 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'

11. Pickle Module selfref_list = [1, 2, 3] output = open('data.pkl', 'wb') pickle.dump(selfref_list, output) data1 = pickle.load(output) When reading a pickle-containing file, you should open the file in binary mode because you can't be sure if the ASCII or binary format was used

12. Reference • http://docs.python.org/tutorial/inputoutput.html • http://www.tutorialspoint.com/python/string_rjust.htm • http://diveintopython.org/native_data_types/formatting_strings.html • http://docs.python.org/library/ • http://en.wikibooks.org/wiki/Python_Programming/Input_and_output • http://docs.python.org/release/2.5.2/lib/pickle-example.html