Python Study Notes - File Operations

2020-09-05 922 words 5 minutes

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Contents

File Operations

open

open() is the key function in Python for file operations, with two parameters that need to be set:

Filename - the name of the file, self-explanatory
Mode - determines if the file being opened can be read/written to or has other attributes.

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open('filename','mode')

Reading

Open a file in read-only mode:

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f = open("filename.txt")

This is equivalent to:

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f = open("filename","rt")

“r” indicates to read

“t” indicates that the file is text, this is the default setting for the function, so it can be omitted.

Here’s a list from w3schools:

There are four different methods (modes) for opening a file:

“r” - Read - Default value. Opens a file for reading, error if the file does not exist

“a” - Append - Opens a file for appending, creates the file if it does not exist

“w” - Write - Opens a file for writing, creates the file if it does not exist

“x” - Create - Creates the specified file, returns an error if the file exists

In addition you can specify if the file should be handled as binary or text mode

“t” - Text - Default value. Text mode

“b” - Binary - Binary mode (e.g. images)

For example, let’s say we have a file:

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/home/weepingdogel/test.txt
---
Hello!I love Python.

We can read the file without specifying the mode parameter:

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f = open('test.txt')
print(f.read())

Output:

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weepingdogel@WeepingDogel ~> python test.py
Hello!I love Python.

Or we can specify it:

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f = open('test.txt', 'rt')
print(f.read())

Output:

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weepingdogel@WeepingDogel ~> python test.py
Hello!I love Python.

Reading lines

File:

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/home/weepingdogel/test.txt
---
Hello!I love Python.
Have a nice day!
Good luck!

When we encounter a multiline file, we can choose to read only one line at a time using f.readline()

For example:

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f = open('test.txt')
print(f.readline())

Output:

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weepingdogel@WeepingDogel ~> python test.py
Hello!I love Python.

If we need two lines:

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f = open('test.txt')
print(f.readline())
print(f.readline())

Output:

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weepingdogel@WeepingDogel ~> python test.py
Hello!I love Python.

Have a nice day!

If we need three lines:

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f = open('test.txt')
print(f.readline())
print(f.readline())
print(f.readline())

Output:

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weepingdogel@WeepingDogel ~> python test.py
Hello!I love Python.

Have a nice day!

Good luck!

This usage reads line by line and prints with line breaks.

You may need it when reading configuration files…

Of course, we can also use a for() loop to read all lines at once:

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f = open('test.txt')
for x in f:
    print(x)

Output:

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weepingdogel@WeepingDogel ~> python test.py
Hello!I love Python.

Have a nice day!

Good luck!

I think using for is more efficient…

Closing files

Nothing much to say here…

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f = open('test.txt')
print(f.read())
f.close()

The effect is similar to the previous example.

I won’t provide debugging results below, it’s too late.

Creating

“x” indicates creating a new file. If the specified filename already exists, an error will be returned.

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f = open("test.txt","x")

Try it out yourself, nothing much else.

Writing to a file

The character 'a' represents adding content to an existing file without deleting or overwriting its original contents.

For example:

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f = open("test.txt","a")
f.write("加入内容 / content added.")

The above string will be added to the file.

The character 'w' represents overwriting the file, which will replace any existing content.

For example:

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f = open("test.txt", "w")
f.write("加入内容 / content added.")

In this case, only the string specified will exist in the file.

Deleting a file

You need to use the os module and its os.remove() function. Simply type import os to import it.

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

Classic example

Check if a file exists, delete it if it does, or display a message if it doesn’t.

Deleting a directory

Use os.rmdir().

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import os
os.rmdir("foldername")

Conclusion

These are the basics of file read/write operations that you should know.

If you’re having trouble understanding, you can try running the following code with different open() mode parameters.

Summary code:

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import os
import datetime

def sign():
    # Program identification
    print(
'''
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╱╱╱╱╱╱╱╱╱╱╱┃┃╱╱╱╱╱╭━╯┃╱╱╱╱╱╱╭━╯┃
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'''
    )

def filecrt(filename):
    # File creation
    if os.path.exists(filename): # Check if the file exists
        print(str(datetime.datetime.now()) + ": The file already exists")
        return 0
    else:
        f = open(filename,'x')
        f.close()
        print(str(datetime.datetime.now()) + ": Created file: " + filename)
        return 1

def filewrt(filename):
    fruits = ['apple', 'banana', 'strawberry','orange'] # Specify the contents to be written
    # File write operation
    f = open(filename, 'w')
    for fruit in fruits:
        f.write(fruit + '\n')
        print(str(datetime.datetime.now()) + ": Writing: " + fruit)
    f.close()

def filedel(filename):
    # Delete file operation
    if os.path.exists(filename):
        os.remove(filename)
        print(str(datetime.datetime.now()) + ": Deleted file: " + filename)
    else:
        print(str(datetime.datetime.now()) + ": " + filename + " does not exist")

def fileread(filename):
    print(str(datetime.datetime.now()) + ": Reading..." )
    f = open(filename,'r')
    print("-" * 5 + " File contents " + "-" * 5 + "\n")
    print(f.read())
    print("-" * 5 + " File contents " + "-" * 5 + "\n")

sign()

if filecrt("test.txt") == 0:
    fileread("test.txt")
    filedel("test.txt")
else:
    filewrt("test.txt")
    fileread("test.txt")
    filedel("test.txt")