Notes of Py for informatics 2

Chapter 7 Files

File PRocessing

www.py4inf.com/code/mbox-short.txt

open() returns a "file handle" - a variable used to perform Operations on the file

handle is a connection. doesn't actually have data

kind of like "File -> Open" in a Word Processor

handle = open(filename, mode) #filename is a stringfhand = open('mbox.txt','r') #'r':reading,'w':writing
The newline Character
stuff = 'Hello/nWorld'  #/n means new line, and it is counted as a characterprint stuffprint len(stuff)output:
HelloWorld11
Counting Lines in a File
xfile = open('/Users/huyifan/documents/mbox.txt')count = 0for line in xfile:    count = count + 1print 'Line Count:',countimportant! A file handle ope for read can be treated as a sequence of strings where each line in the file is a string in the sequence
Reading the *Whole* File
Use read() to read the whole file (newlines and all) into a single string.
xfile = open('/Users/huyifan/documents/mbox.txt')inp = xfile.read()print len(inp)print inp[:20]output:
94625From stephen.marquarSearching Through a File
for line in xfile:    if line.startswith('Author'):        print lineoutput:
Author: stephen.marquard@uct.ac.za                                     #why there are black lines? because of /n !Author: louis@media.berkeley.eduAuthor: zqian@umich.eduAuthor: rjlowe@iupui.eduAuthor: zqian@umich.eduAuthor: rjlowe@iupui.eduWe can use the rstrip() or lstrip().
for line in xfile:    if line.startswith('A'):        print line.rstrip()output:
Author: david.horwitz@uct.ac.zaAuthor: stephen.marquard@uct.ac.zaAuthor: louis@media.berkeley.eduAuthor: louis@media.berkeley.eduAuthor: ray@media.berkeley.eduAuthor: cwen@iupui.eduAuthor: cwen@iupui.eduAuthor: cwen@iupui.edu
Skipping with continue
for line in xfile:    if not line.startswith('Author'):  #or: if not '@media.berkeley.edu' in line:        continue   #process our 'interesting lines'    print line.rstrip()The output will be the same.
Bad File Names
fname = raw_input('Enter the file name:')try:    fhand = open(fname)except:    print 'File cannot be openned:', fname    exit()count = 0for line in fhand:    if line.startswith('Author:'):        count = count + 1print 'There were', count, 'pieces of important information'
Chapter 8 Lists
8.1 basic understanding and operationsA list is a kind of collection.
A collection allows us to put many values in a single "variable"
like: 
friends = ['Joe','Joey','Joseph']
A list element can be any Python object - even another list
print [[1,2],3,4]
A list can be empty
print []Strings are "immutable" - we cannot change change the contents of a string - we must make a new string to make any change
Lists are "mutable" - we can change an element of a list using the index operator
len()
print len([[1,2],3,4])Using the range function: generate a list
print range(4)  # often be used as: for i in range(len(friends))    friend[i]output:
[0, 1, 2, 3]
Concatenating list using +
print range(4)+[2,3]output:
[0, 1, 2, 3, 2, 3]Slice lists: like 'strings'
8.2 Build-in Functions
Building a list from scratch: xxx.append()
stuff = list()stuff.append('book')stuff.append(99)print stuff
output:
['book', 99]xxx in xxxx?
Order the list
friends = ['Joey','Joe','Joseph','Mike','Ellen']friends.sort()print friendsoutput:
['Ellen', 'Joe', 'Joey', 'Joseph', 'Mike']   #important: it has changed itself!The sort method (unlike in strings) means "sort yourself"
other useful build-in functions
len(numbers), max(numbers), min(numbers), sum(numbers)Strings -> Lists: Use split()/split(';')/... to break a string into parts and produces a list of strings. (leave out all the comma/space..)
abc = 'With three words'stuff = abc.split()print stuffprint len(stuff)output:
['With', 'three', 'words']3
abc = 'With;three;words'stuff1 = abc.split()stuff2 = abc.split(';')   #important!print stuff1print stuff2print len(stuff1)print len(stuff2)output:
['With;three;words']['With', 'three', 'words']13