Files

Files are the main ingredients of our programs. Our scripts are files; we may have input files; we will usually want some kind of output file. We can manipulate files in Python without having to go through the operating system’s user interface.

Files, Folders, and Paths

Three major operating systems are in use today; Windows, MacOS, and Linux. Each one does things a little differently.

Paths and Platforms

The location of a file is specified by its path. The exact format of the path varies somewhat by operating system. Python uses forward slashes to separate folders, even on Windows where the backslash (\) is “native.”

Python tends to be rooted in the Linux operating system so some of the vocabulary comes from there. “Folders” in Windows and MacOS are called directories in Linux. The full path to a file is the tree of all folders/directories that must be traversed to reach it.

#Windows
filename="C:/Users/You/Desktop/Python Programs/myscript.py"
#MacOS
filename="/Users/You/Desktop/Python Programs/myscript.py"
#Linux
filename="/home/you/python_programs/myscript.py"

We can use the os module to make our scripts a little more platform-independent. The os module provides access to some basic operating-system functionality, particularly those related to files, in an interface that is uniform across different systems.

In this example, the expanduser function will get the home directory of the user running the script. For this we must use the path submodule of os.

import os
home_dir=os.path.expanduser('~')

The tilde ~ stands for the home directory in any operating system.

We can use path.join to concatenate directories and paths into complete file names. On Windows, path.join understands both forward and backward slashes.

#Windows
dir="Desktop\Python Programs"
#MacOS
dir="Desktop/Python Programs"
#Linux
dir="python_programs"
#All
script="myscript.py"
filename=os.path.join(home_dir,dir,script)
f=open(filename)

The os module can perform basic file and directory manipulations in a way that is appropriate for each operating system.

Changing and Creating Directories

When we run a script, the path from which it is run is the current working directory. Note that JupyterLab and IDEs may set the current working directory their own way. We can get it through the os module and we can change it.

mycwd=os.getcwd()
newpath="/home/you/some_dir"
os.chdir(newpath)

To create a new directory, use mkdir

os.mkdir(new_dir) #relative to CWD
os.mkdir(fullpath)   #full path

Listing Files

We can list the files in a directory with listdir from os. It returns a list.

files=os.listdir() #current working directory
input_directory=os.path.listdir(input_path) #using a full path

We can check whether an item is a directory with os.path.isdir(), which can also check whether it exists. Similar for os.path.isfile().

#Starting from CWD
for file in os.listdir():
    if os.path.isdir(file):
        print("{} is a directory".format(file))
    elif os.path.isfile(file):
        print("{} is a file".format(file))
    else:
        print("{} is neither a file nor a directory".format(file))

If we do not need a list of the files but only an iterator, we can use scandir. Scandir returns an object, not a string, so we must extract the parts we need. The advantage to scandir is that it can be faster if we need to test any attributes of the file. The with statement is intended to handle any exceptions.

import os
#CWD
with os.scandir() as it:
    for entry in it:
        if entry.is_file():
            print("{} is a file".format(entry.name))
        elif entry.is_dir():
            print("{} is a directory".format(entry.name))
        else:
            print("{} is not a file or a folder".format(entry.name))
  

Copying and Moving Files

Another module, shutil, allows us to move and copy files and perform other basic operations on them.

import shutil
shutil.copy(source,destination)
shutil.move(source,destination)

In the above, both source and destination should be either strings or should be paths created by some function such as os.path.join.

For more details about the os module, see its documentation.

Opening a File

Before anything can be done with a file we must open it. This attaches the file name to the program through some form of file descriptor, which is like an identifier for the file. Once opened we do not refer to the file by its name anymore, but only via the ID.

We open a file and associate an identifier with it by means of the open statement.

fp=open("filename")
fin=open("filename","r")
fout=open("filename","w")

The default is read-only. Adding the r makes this explicit. To open for writing only, add the w. This overwrites the file if it already exists. To append to an existing file, use a in place of w. Both w and a will create the file if it does not exist. To open for both reading and writing, use r+; in this case the file must exist. To open it for reading and writing even if it does not exist, use w+.

Closing Files

When you have completed all operations on a file you should close it.

fin.close()
fout.close()

Files will be automatically closed when your script terminates, but best practice is to close each file yourself as soon as you are done with it. You must close a file if you intend to open it later in a different mode. You cannot reopen a file using an active file descriptor. You must first close it.

Exercise

All three major operating systems have a Documents directory by default in their desktop environments. Write a script to

1. change the working directory to the Documents directory. Use a general way to construct the path.
2. List all the files in the directory. Use a string function to check whether any end in “.txt.” OK to use listdir.
3. Make a new folder “MyTestFolder” in the Documents directory. Check first whether it already exists.
4. Open a file “new_file.txt” for writing.
5. Use the following line to write a little text into it. Replace “f” with your choice of file identifier. f.write(“Here are some words for this file.\n”)
6. Close the file.
7. Move the file into “MyTestFolder.”

import os
import shutil

home_dir=os.path.expanduser('~')
doc_dir="Documents"
doc_path=os.path.join(home_dir,doc_dir)

os.chdir(doc_path)

for file in os.listdir():
    print(file)
    if file.endswith("txt"):
        print("File {} is a text file.".format(file))

new_folder="MyTestFolder"
if not os.path.isdir(new_folder):
    os.mkdir(new_folder)

new_file="new_file.txt"
f=open(new_file,"w")

f.write("Here are some words for this file.\n")
f.close()

shutil.move(new_file,new_folder)