Python Programming/Files

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[edit] File I/O

Read entire file:

inputFileText = open("testit.txt", "r").read()
print inputFileText

In this case the "r" parameter means the file will be opened in read-only mode.

Read certain amount of bytes from a file:

inputFileText = open("testit.txt", "r").read(123)
print inputFileText

When opening a file, one starts reading at the beginning of the file, if one would want more random access to the file, it is possible to use seek() to change the current position in a file and tell() to get to know the current position in the file. This is illustrated in the following example:

>>> f=open("/proc/cpuinfo","r")
>>> f.tell()
0L
>>> f.read(10)
'processor\t'
>>> f.read(10)
': 0\nvendor'
>>> f.tell()
20L
>>> f.seek(10)
>>> f.tell()
10L
>>> f.read(10)
': 0\nvendor'
>>> f.close()
>>> f
<closed file '/proc/cpuinfo', mode 'r' at 0xb7d79770>

Here a file is opened, twice ten bytes are read, tell() shows that the current offset is at position 20, now seek() is used to go back to position 10 (the same position where the second read was started) and ten bytes are read and printed again. And when no more operations on a file are needed the close() function is used to close the file we opened.

Read one line at a time:

for line in open("testit.txt", "r").readlines():
    print line

In this case readlines() will return an array containing the individual lines of the file as array entries. Reading a single line can be done using the readline() function which returns the current line as a string. This example will output an additional newline between the individual lines of the file, this is because one is read from the file and print introduces another newline.

Write to a file requires the second parameter of open() to be "w", this will overwrite the existing contents of the file if it already exists when opening the file:

outputFileText = "Here's some text to save in a file"
open("testit.txt", "w").write(outputFileText)

Append to a file requires the second parameter of open() to be "a" (from append):

outputFileText = "Here's some text to add to the existing file."
open("testit.txt", "a").write(outputFileText)

Note that this does not add a line break between the existing file content and the string to be added.

[edit] Testing Files

Determine whether path exists:

import os
os.path.exists('<path string>')

When working on systems such as Microsoft Windows(tm), the directory separators will conflict with the path string. To get around this, do the following:

import os
os.path.exists('C:\\windows\\example\\path')

A better way however is to use "raw", or r:

import os
os.path.exists(r'C:\windows\example\path')

But there are some other convenient functions in os.path, where path.code.exists() only confirms whether or not path exists, there are functions which let you know if the path is a file, a directory, a mount point or a symlink. There is even a function os.path.realpath() which reveals the true destination of a symlink:

>>> import os
>>> os.path.isfile("/")
False
>>> os.path.isfile("/proc/cpuinfo")
True
>>> os.path.isdir("/")
True
>>> os.path.isdir("/proc/cpuinfo")
False
>>> os.path.ismount("/")
True
>>> os.path.islink("/")
False
>>> os.path.islink("/vmlinuz")
True
>>> os.path.realpath("/vmlinuz")
'/boot/vmlinuz-2.6.24-21-generic'

[edit] Common File Operations

To copy or move a file, use the shutil library.

import shutil
shutil.move("originallocation.txt","newlocation.txt")
shutil.copy("original.txt","copy.txt")

To perform a recursive copy it is possible to use copytree(), to perform a recursive remove it is possible to use rmtree()

import shutil
shutil.copytree("dir1","dir2")
shutil.rmtree("dir1")

To remove an individual file there exists the remove() function in the os module:

import os
os.remove("file.txt")