Python For Informatics Chapter 7

Front 1

opening a file

Back 1

Opening the file communicates with your operating system which knows where the data for each file is stored. When you open a file, you are asking the operating system to find the by name and make sure the file exists.

fhand = open('filename.txt')

print fhand

Front 2

newline character

Back 2

In Python we use backslash--n in string constants.

>>> stuff = 'X\nY'
>>> print stuff
X
Y

Front 3

startswith()

Back 3

Checks what a string starts with like this

fhand = open('mbox-short.txt')
for line in fhand:
if line.startswith('From:') :
print line

would print all lines starting with From:

Front 4

Writing files

Back 4

To write a file you have to open it with mode 'w' as a second parameter.

>>> fout = open('output.txt', 'w')
>>> print fout

The write method of the file handle object puts data into the file.
>>> line1 = 'This here's the wattle,\n'
>>> fout.write(line1)
Again, the file object keeps track of where it is, so if you call write again, it adds
the new data to the end.

Front 5

repr()

Back 5

Takes any object as an argument and returns a string representation of the object. For strings, it represents whitespace characters with backslash sequences:

>>> s = '1 2\t 3\n 4'
>>> print s
1 2 3
4

>>> print repr(s)
'1 2\t 3\n 4'