8. Files

8.1. Quick example

In order to open a file (to read or to write), use the open() function:

# here we use the write mode (w) to open a file to write in
f = open('file.txt', 'w')
f.write("something on a single line")
f.close()

# here we use the read-only mode (r) to read a file
f = open('file.txt', 'r') # by default the mode is read-only
while True:
    line = f.readline()
    if len(line) == 0:
        break
    print(line), # note the comma to avoid automatic new line
f.close()

The mode can be ‘w’, ‘r’, ‘a’ for reading (default), writing or appending.

The file will be created if it doesn’t exist when opened for writing or appending; it will be truncated when opened for writing. Add a ‘b’ to the mode for binary files. Add a ‘+’ to the mode to allow simultaneous reading and writing.

You can also provide a third argument for buffering. If ommitted (or 0), no buffer is used.; 1 means buffer one line at a time; larger numbers specify the buffer size.

Todo

usage of with.

8.2. Attributes

The are attributes to access to the name and mode of a file handler as well as its closed:

>>> f.name
'/tmp/file.txt'
>>> f.mode
'w'
>>> f.closed
True

8.3. Methods to read a file

You can read a file line by line with readline:

>>> line = f.readline()

If EOF is reached, line is an empty string, which is False. So to read all the lines, you can type:

>>> for line in f.readline()
...     # do something with line

If the file is not too large, you could read all the lines in one go to store them in a list:

>>> lines = f.readlines()

Or you can read the entire file as a single string:

>>> data =  f.read()

Warning

The newline character ‘n’ is included in all the line

Since files are also iterators, a fourth way to read a file is to iterate it:

>>> for line in f:
...     print line,

Note

the comma after the print statement is required. Indeed each line already contain the n character.

If your file is large, you can use the file.xreadlines() function that use less memory than the readline(s) functions. xreadlines returns the same list as readlines but it does not build the entire list at the same time, only when it is needed.

To read only N characters, use an optional argument to the read function.

8.4. Methods to write in a file

The write and writelines functions are the functions to be used to write in a file. To write string in a file you should use write(). To write a list of strings, use writelines().

lst = ['a', 'b', 'c']
f.writelines(lst)

f.write("single string")

writelines does not qdd newlines at the end of each strings so you should add them yourself. You could do

f.writelines([x+"\n" for x in lst])

Python performs buffering. So, data that you write may not actually appear in the file unless you can f.flush or f.close method. Any buffered data not yet written is written to the file when flush is called. Closing file is not stricly needed since Python closes files automatically when your script finishes. Hozever there are 2 good reasons to close files yourself: it saves memory and you will not write in the file by mistakes since a closed file cannot be manipulated anymore.

8.5. Changing position in a file

The file.seek() Method is used to move the cursor position to different locations within a file.

The file.tell() Method displays the current position of the cursor in a file from the start of the file. The first chracter is at position 0.

>>> f = open("temp.txt", "w")
>>> f.write("first line\n")
>>> f.write("second line\n")
>>> f.write("third line\n")
>>> f.close()
>>> f = open("temp.txt", "r")
>>> f.tell()
0
>>> f.readline()
'first line\n'
>>> f.tell()
11

Now you can rewind back to the beginning of the file with file.seek():

>>> f.seek(0)
>>> f.readline()
'first line\n'

The argument can be set to 0 as above, or 1 to use the current position or 2 to use the end of the file as the reference position.

8.6. Other methods

There a are few other methods available when manipulating files.

You can truncate a file but you must open as read/write:

>>> f = open("temp.txt", "r+")
>>> f.readline()
>>> f.tell()
11
>>> f.truncate(11)

Files have the iteratr method implemented, so meth:file.next returns the next value from a file. It is equivalent to file.readline().

You can also get file information using file.fileno() to obtain the file descriptor, zhich is used at low-level file operations.

The file.softspace is used to indicate whether a space character needs to be printed before another value when using the print statement. You should not use it unless you want to implement a type in C language where you will have to provide the softspace attribute.

The file.isatty() method returns True if the file is connected to a tty device.

The encoding that a file uses is stored in file.encoding. When Unicode strings are written to a file, they will be converted to byte strings using this encoding.

Finally, there is a unicode error handler used along the encoding that is stored in file.errors

8.7. Other way to open files

You can play with the os module with os.fdopen(), popen, tmpfile, popen2, popen3, popen4 functions. See The os module (and sys, and path)