Python Input and Output Output Formatting Two methods

Python Input and Output

Output Formatting • Two methods to format output 1. Perform all string formatting yourself 2. Use the str. format() method

Converting Values to Strings • Use either the str() or repr() functions • str() will return a “human-readable” representation • repr() will return representations that may be read from the interpreter • If str() is not defined for a given object, str() will return the same value as repr()

str() and repr() >>> s = “hello, worldn” >>> print str(s) hello, world >>> print repr(s) ‘hello, worldn’

Justifications • rjust() will right-justify a string of a given width and pad it with spaces on the left

rjust() example >>> for x in range(1, 6): . . . print repr(x). rjust(1), . . . print repr(x * x). rjust(2), . . . print repr(x * x). rjust(3) 1 1 1 2 4 8 3 9 27 4 16 64 5 25 125

Other Similar Methods • ljust() – for left justification • center() – for center justification • zfill() – will pad a numeric string with 0’s on the left • example of zfill(): >>> '13'. zfill(5) '00013' >>> '-13'. zfill(5) '-0013'

str. format() >>> print 'This {0} uses format {1}'. format('string', 'fields') This string uses format fields >>> print 'This {food} is {adjective}!'. format(food='spam ', adjective='great') This spam is great!

!s and !r • Use !s inside brackets to apply str() to the value before it is formatted • Use !r to apply repr()

Additional Formatting • Use ‘: ’ following the field name for additional formatting • For example, to truncate Pi to three places: >>> print 'PI is about {0: . 3 f}. '. format(math. pi) PI is about 3. 142.

• An integer after the ‘: ’ will cause the field to be a minimum number of characters wide >>> table = {'x': 120, 'y': 121, 'z': 122} >>> for char, value in table. items(): . . . print '{0: 10} ==> {1: 10 d}'. format(char, value) x ==> 120 y ==> 121 z ==> 122

Reading and Writing Files • open() – returns a file object • Commonly called with two arguments 1. filename 2. mode >>> f = open('/temp/myfile', 'w') • close() – closes a file, freeing up system resources

Modes Mode string Description ‘r’ Read only (default if not specified) ‘w’ Writing (will erase existing file with same name ‘a’ Append ‘r+’ Reading and Writing

Reading • filename. read(size) – read and return a specified number of bytes – filename. read() – read the entire file • filename. readline() – reads a single line from the file – keeps the ‘n’ at the end, unless the end of file has been reached • filename. readlines() – returns a list containing each line in the file

Writing • filename. write(string) – writes the contents of the string to the file – must convert non-string values to strings first with either str() or repr()

Other File Methods • filename. tell() – returns an integer representing the current position in the file – measured in bytes from the beginning of the file

• filename. seek(offset, from_what) – computes a position by adding the offset to a reference point (from_what) • from_what may have 1 of 3 values from_what Description 0 Beginning of file (default) 1 Current position 2 End of file (use a negative offset)

Any Questions? Reference: http: //docs. python. org/tutorial/inputoutput. html