Python Programming/Files< Python Programming
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"): 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.
Since Python 2.5, you can use with keyword to ensure the file handle is released as soon as possible and to make it exception-safe:
with open("input.txt") as file1: data = file1.read() # process the data
Or one line at a time:
with open("input.txt") as file1: for line in file1: print line
Related to the with keywords is Context Managers chapter.
Determine whether path exists:
import os os.path.exists('<path string>')
When working on systems such as Microsoft Windows™, 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
import os os.path.exists(r'C:\windows\example\path')
But there are some other convenient functions in
os.path.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'
Common File OperationsEdit
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
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")
Files can be found using glob:
glob.glob('*.txt') # Finds files in the currect directory ending in dot txt glob.glob('*\\*.txt') # Finds files in any of the direct subdirectories # of the currect directory ending in dot txt glob.glob('C:\\Windows\\*.exe') for fileName in glob.glob('C:\\Windows\\*.exe'): print fileName glob.glob('C:\\Windows\\**.exe', recursive=True) # Py 3.5: ** allows recursive nesting
The content of a directory can be listed using listdir:
filesAndDirectories=os.listdir('.') for item in filesAndDirectories: if os.path.isfile(item) and item.endswith('.txt'): print "Text file: " + item if os.path.isdir(item): print "Directory: " + item
Getting a list of all items in a directory, including the nested ones:
for root, directories, files in os.walk('/user/Joe Hoe'): print "Root: " + root # e.g. /user/Joe Hoe/Docs for dir1 in directories: print "Dir.: " + dir1 # e.g. Fin print "Dir. 2: " + os.path.join(root, dir1) # e.g. /user/Joe Hoe/Docs/Fin for file1 in files: print "File: " + file1 # e.g. MyFile.txt print "File 2: " + os.path.join(root, file1) # e.g. /user/Joe Hoe/Docs/MyFile.txt
Above, root takes value of each directory in /user/Joe Hoe including /user/Joe Hoe itself, and directories and files are only those directly present in each root.
Getting a list of all files in a directory, including the nested ones, ending in .txt, using list comprehension:
files = [os.path.join(r, f) for r, d, fs in os.walk(".") for f in fs if f.endswith(".txt")] # As iterator files = (os.path.join(r, f) for r, d, fs in os.walk(".") for f in fs if f.endswith(".txt"))
Getting current working directory:
Changing current working directory: