File Handling with Python

# a look at file handling in Python
# tested with Python24    vegaseat   29sep2005

# set up a test string
str1 = """There is no ham in hamburger.
Neither apple nor pine are in pineapple.
Boxing rings are square.
Writers write, but fingers don't fing.
If a vegetarian eats vegetables, what does a humanitarian eat?
Overlook and oversee are opposites.
Slim chance and fat chance are the same.
A house can burn up as it burns down.
"""

# more to test file appending
str2 = """An alarm goes off by going on.
Fill in a form by filling it out.
"""

# let's create our test file by writing the test string to the working folder/directory with write()
# modifier "w" is for writing text, use "wb" for binary data like images
fout = open("English101.txt", "w")
fout.write(str1)
fout.close()

# read back the entire test string as a string with read()
# "r" is for reading text, use "rb" for binary data like images
fin = open("English101.txt", "r")
str3 = fin.read()
fin.close()
print "Contents of file English101.txt:"
print str3

# a similar read using try/except error handling
# tested this out by deliberately changing the filename
filename = "English102.txt"
try:
    fin = open(filename, "r")
    str3 = fin.read()
    fin.close()
    print "Contents of file %s:" % filename
    print str3
except IOError:
    print "File %s does not exist!" % filename

print

# append more text to an existing file with modifier "a"
fout = open("English101.txt", "a")
fout.write(str2)
fout.close()

# read the appended text file as a list of lines with readlines()
fin = open("English101.txt", "r")
lineList = fin.readlines()
fin.close()
print "Contents of appended file (first option):"
for line in lineList:
    print line,
    
print

# a short-form to do this, uses readlines() internally
# the comma at the end of print takes care of the extra newline character
# note: Python does clean up and closes the file for you
print "Contents of appended file (second option):"
for line in open("English101.txt", "r"):
    print line,

print

# similar to above, but creating a list with list comprehension
line_list = [line for line in open("English101.txt", "r")]
print "A list of the text lines:"
print line_list   # test

print

# read just one line of text at a time
print "The first two lines:"
fin = open("English101.txt", "r")
print "Line 1 =", fin.readline(),
print "Line 2 =", fin.readline()   # etc.
fin.close()

# show just the last line of text
fin = open("English101.txt", "r")
lineList = fin.readlines()
fin.close()
print "Last line =", lineList[-1],
print "Total lines =", len(lineList)

# the whole thing can be simplified (more cryptic though)
lastLine = file("English101.txt", "r").readlines()[-1]
print "Last line =", lastLine

# do some random access of the file
fin = open("English101.txt", "r")
# seek index is zero based, so the 10th character would be position 9
fin.seek(9)
print "From character 10 to end of line =", fin.readline()
print "End if this line is at character =", fin.tell()
print
num = 16
pos = 80
print "Read %d characters starting at position %d:" % (num, pos)
fin.seek(pos)
print fin.read(num)
fin.close()

print

# read a particular line, lineNumber is zero based
import linecache
lineNumber = 5
partLine = linecache.getline("English101.txt", lineNumber)
print "Line %d = %s" % (lineNumber, partLine)

print

# processing the lines as you read them in and forming a list
# using list comprehension
list2 = [line.replace(".", "!") for line in open("English101.txt", "r")]

# display the result
print "Processing the lines as you read them in ..."
print "Each period has been replaced with an exclamation mark:"
for line in list2:
    print line,
   
print; print

# print to a file (a different option of write)
fout = open( "test1.txt", "w" )
print >>fout, "I love Monte Python!"
fout.close()

# a file exists if you can open and close it
def exists(filename):
    try:
        f = open(filename)
        f.close()
        return True
    except:
        return False

# what does file object fin look like?
filename = 'test1.txt'
if exists(filename):
    fin = open(filename)
    print "file object =", fin
    print "file content =", fin.read()
    fin.close()
else:
    print "File %s does not exist!" % filename

print

# for large text files you can write and read a zipped file (PKZIP format)
# notice that the syntax is mildly different from normal file read/write
import zipfile
zfilename = "English101.zip"
zout = zipfile.ZipFile(zfilename, "w")
zout.writestr(zfilename, str1 + str2)
zout.close()
# read the zipped file back in
zin = zipfile.ZipFile(zfilename, "r")
strz = zin.read(zfilename)
zin.close()
print "Testing the contents of %s:" % zfilename
print strz

print

# read a binary image file, pick something you have ...
# (also shows exception handling)
filename = "Moo.jpg"
try:
    fin = open(filename, "rb")
    data = fin.read()
    fin.close()
    print "This is a hex-dumb of %s:" % filename
    for c in data:
        print "%02X" % ord(c),
    print
except IOError:
    print "Binary File %s not found" % filename
    #raise SystemExit  # optional exit

print

# below is a typical Python dictionary object of roman numerals
romanD1 = {'I':1,'II':2,'III':3,'IV':4,'V':5,'VI':6,'VII':7,'VIII':8,'IX':9,'X':10}

# to save a Python object like a dictionary to a file
# and load it back intact you have to use the pickle module
import pickle
print "The original dictionary:"
print romanD1
file = open("roman1.dat", "w")
pickle.dump(romanD1, file)
file.close()
# now load the dictionay object back from the file ...
file = open("roman1.dat", "r")
romanD2 = pickle.load(file)
file.close()
print "Dictionary after pickle.dump() and pickle.load():"
print romanD2

print

# module StringIO allows you to treat a data stream like a file
# if you do a lot of processing, memory streams are much faster then file streams
# (StringIO.StringIO is a class that can be inherited in a class of your own)
print "You can stream text/data to memory ..."
import StringIO
stream1 = StringIO.StringIO(str2)  # use the string str2 here, or read one in from a file
print str2

# show the memory where the object is located
print stream1

print

print "... and use stream like a file:"
print stream1.readline()

# reset the stream to zero (beginning) and read all lines
stream1.seek(0)
list1 = stream1.readlines()
print "All lines from beginning:"
for item in list1:
    print item,
print
# reset the stream to position 9 and read the next 20 characters
stream1.seek(9)
print "Read 20 characters starting at position 9:"
print stream1.read(20)

# finally close the stream
stream1.close()