Download Programming in Python

File handling The sys module
File handling The sys module
Opening files
File handling
I
The call open(filename, mode=’r’) will return a file
object, whose type is file
I
This file object can be used to refer to a file on disk. For
example, when we want to read from or write to a file, we can
use the methods read and write of the file object
I
After the file object is no longer needed, a call to the close
method should be made
The sys module
Jarkko Toivonen (CS Department)
Programming in Python
1 / 13
Jarkko Toivonen (CS Department)
File handling The sys module
File open modes 2
I
We can control what kind of operations we can perform on a
file with the mode parameter of open function
I
Different options include opening a file for reading or write,
whether the file should exists already or be created with the
call to open, etc
I
Here’s a list of all the opening modes:
read-only mode, file must exist
write-only mode, creates, or overwrites an existing file
write-only mode, write always appends to the end
read/write mode, file must already exist
read/write mode, creates, or overwrites an existing file
read/write mode, write will append to end
Jarkko Toivonen (CS Department)
2 / 13
File handling The sys module
File open modes 1
r
w
a
r+
w+
a+
Programming in Python
Programming in Python
3 / 13
I
The mode specification can optionally end with b or t
character, denoting binary and text modes, correspondingly
I
On Unix, there’s no difference between these modes
I
On Windows, when using text mode, the line ending
characters are interpreted as \n. This is supposed to make file
handling more portable
Jarkko Toivonen (CS Department)
Programming in Python
4 / 13
File handling The sys module
File handling The sys module
Some common file object methods
I
read(size) will read size characters as a string from a file
I
write(string) will write string to a file
I
Seeking and telling the position in a file 1
I
Normally all these read and write operations work in
consequtive manner
readline() will read a string until and including the next
newline character is met
I
That is, the next read operation will continue from the file
position where the last read operation finished
I
readlines() will return a list of all lines of a file
I
I
writelines(lst) will write a list of lines to a file
And, the next write operation will continue from the position
where the previous write finished
I
flush() will try to make sure that the changes made to a file
are written to disk immediately
I
This behaviour can be changed with the seek method:
f.seek(pos, how=0)
Jarkko Toivonen (CS Department)
Programming in Python
5 / 13
Jarkko Toivonen (CS Department)
File handling The sys module
The pos argument is an integer that refers to a file position
starting from the reference point how
I
The reference point is either the start of the file (how==0),
current position (how==1), or the end of the file (how==2)
I
Note that the pos parameter can also be negative
I
The method tell will return an integer that tells the current
position from the start of the file
Jarkko Toivonen (CS Department)
Programming in Python
6 / 13
File handling The sys module
Seeking and telling the position in a file 2
I
Programming in Python
Iterating a file object
I
We can iterate through all the lines of a file f using the
following syntax:
for line in f:
# process line
7 / 13
I
The file f needs to be open for text-mode reading
I
This is an efficient way of processing a file
Jarkko Toivonen (CS Department)
Programming in Python
8 / 13
File handling The sys module
File handling The sys module
Standard file objects 1
I
Standard file objects 2
Python has automatically three file objects open:
I
I
I
sys.stdin for standard input
sys.stdout for standard output
sys.stderr for standard error
I
These standard file objects are meant to be a basic
input/output mechanism in textual form
I
To read a line from a user (keyboard), you can call
sys.stdin.readline()
I
The destinations of the file objects can be changed to point
somewhere else than the usual keyboard and monitor
I
To write a line to a user (monitor), call
sys.stdout.write(line)
I
Very often these are redirected to some files
I
For example, it is usual to point the stderr to a file where all
error messages are logged
I
The standard error is meant for error messages only, even
though often its output goes to the same destination as
standard output
Jarkko Toivonen (CS Department)
Programming in Python
9 / 13
Jarkko Toivonen (CS Department)
File handling The sys module
10 / 13
File handling The sys module
Command line arguments to the program
I
Programming in Python
Command line arguments to the program
Let’s say you start your program from the operating system,
for example, with
python progname.py param1 param2 param3
I
The element sys.argv[0] is the name of the program
I
Python will start executing your program progname
I
I
You can access the command line parameters param1,
param2, and param3 from the sys module’s attribute argv,
which is a list
The rest of the elements sys.argv[1:] are the command
line parameters given to the program
I
The command line parameters can be handled manually or by
using some dedicated module, like the optparse module
I
In the previous example the statement print sys.argv
would print
[’progname.py’, ’param1’, ’param2’, ’param3’]
Jarkko Toivonen (CS Department)
Programming in Python
11 / 13
Jarkko Toivonen (CS Department)
Programming in Python
12 / 13
File handling The sys module
Other useful sys module attributes
I
The function sys.exit(value) will stop the execution of the
program and return value to the operating system.
I
Usually this value tell whether the program ended successfully,
or if not then the value means the error code
I
A return value zero means success
I
The attribute sys.path contains a list of all those directories
that will be searched for imported modules
I
The first element of this list is an empty string that denotes
the current directory
Jarkko Toivonen (CS Department)
Programming in Python
13 / 13