Kristian Rother at 20:32, 5 February 2013

2013-02-05T20:32:14Z

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{{Cat|Python Commands}}
{{Python Links}}

== Opening files for reading ==
Text files can be accessed using the open() function. It returns an open file from which its contents can be extracted as a string.

<source lang="python">
f = open('my_file.txt')
text = f.read()
</source>

== Opening files for writing ==
Writing text to files is very similar. The only difference is the 'w' parameter.

<source lang="python">
f = open('my_file.txt','w')
f.write(text)
</source>

== Appending to files ==
It is possible to add text to an existing file, too.

<source lang="python">
f = open('my_file.txt','a')
f.write(text)
</source>

== Closing files ==
Closing files in Python is not mandatory.

<source lang="python">
f.close()
</source>

== Tips for reading files: ==
An open file behaves like a list of strings. Thus, it is possible to read it line by line without using s.split('\n'):

<source lang="python">
for line in open('my_file.txt'):
name = line[:10].strip()
print name
</source>

== Tips for writing files: ==
Analogously, if you have data as a list of strings, it can be written to a file as a one-liner. You should remember to have linebreaks:

<source lang="python">
lines = ['first line\n',
'second line\n']
open('my_file.txt','w').writelines(lines)
</source>

== Take care of IOErrors ==
You should keep in the back of your head that opening a non-existing file for reading causes an IOError. Eventually, a try.. except clause is useful, then (but not always).

== Writing directory names in Python ==
When opening files, you can also use full or relative directory names. However, you must replace the backslash '\' by a double backslash '\\' (because '\' is also used for '\n' and '\t').

<source lang="python">
f = open('..\\my_file.txt')
f = open('C:\\python\\my_file.txt')
</source>

== Managing directories ==
With the os module, you can change to a different directory:

<source lang="python">
import os
os.chdir(''..\\python'')
</source>

You can also get a list with all files:

<source lang="python">
os.listdir(''..\\python'')
</source>

The third most important function is for checking whether a file exists:

<source lang="python">
print os.access('my_file.txt')
</source>

== The CSV module ==
An open file behaves like a list of strings. Thus, it is possible to read it line by line without using

<source lang="python">
import csv
reader = csv.reader(open('my_file.csv'))
for row in reader:
print row
</source>

Similarly, tables can be written to files:

<source lang="python">
writer = csv.writer(open('my_file.csv','w'))
writer.writerow(table)
</source>

== Options of CSV file readers/writers ==
Both CSV readers and writers can handle a lot of different formats. Options you can change include:
* delimiter: the symbol separating columns.
* quotechar: the symbol used to quote strings.
* lineterminator: the symbol at the end of lines.

<source lang="python">
reader = csv.reader(open('my_file.csv'),
delimiter='\t', quotechar='”')
</source>

File Operations - Revision history

Kristian Rother at 20:32, 5 February 2013