WHAT A MARVELOUS PRICE SONY https://t.co/dGoKPdwmD7

— Katsuhiro Harada (@Harada_TEKKEN) June 14, 2022

Summary: You can use Python’s pickle library to save dictionary data to a file. Another efficient approach to save dictionary data to a file is to use Python’s built-in JSON package. You can also use simple file handling functions to store dictionary data in a text file directly.

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Problem: Given a Python dictionary. How will you save the data from the dictionary to a file so that it can be loaded to be used later?

You might need the help of persistent storage systems like databases or files to store serialized data structures like arrays, lists, and dictionaries. One of the major reasons behind doing so is databases and files are reusable, i.e., after analyzing the given data, we can store it in the file, and later that data can be read to use in an application.

This article will deal with dictionary data that you can store in a file.

Example: Consider the following dictionary:

d = {'country': 'Germany', 'capital': 'Berlin'}

Challenge: How will you store the key-value pairs of the above dictionary in a file?

Related Tutorials:

• Writing a List to a File in Python
• How to Read a Dictionary from a File
• Correct Way to Write line To File in Python

There are numerous ways to store dictionary data in a file using Python. Let’s have a look at some of them:

Method 1: Using Pickle

• Pickle is a module in Python that uses binary protocols to serialize and de-serialize an object structure. “Pickling” refers to the process of converting a Python object to a byte stream. “Unpickling” is just the reverse operation wherein a byte stream is converted to a Python object. Pickling is also sometimes referred to as serialization.
• A good idea to implement a Pickle file is when you are dealing with sensitive data or when you need to keep a program status across sessions.

Approach:

• Create the pickle file (i.e., filename.pkl) with the help of the open(filename, mode) function. Since we will be storing the data in a pickle file which stores the data as a binary stream, hence, open the file in binary (“wb“) mode.
• Use the pickle.dump(dictionary, filename) method to store/serialize the dictionary data to the file.
• To read data from this file, call the pickle.load(filename) method.
• Note: Remember to close the file.

Code: Let’s visualize the above approach with the help of the following code snippet:

import pickle

d = {'country': 'Germany', 'capital': 'Berlin'}

file = open("dictionary_data.pkl", "wb")

pickle.dump(d, file)

file.close()

file = open("dictionary_data.pkl", "rb")

output = pickle.load(file)

print(output)

file.close()

Output:

Output Console:

{'country': 'Germany', 'capital': 'Berlin'}

Method 2: Using json

Approach:

• Open the file in write mode by calling open('filename','mode').
• Use the json.dump(dictionary, filename) function to convert the dictionary data to the json format and write it to the file.
• To read the data from this file, read the data from it by calling the file.read() function.
• Note: Remember to close the file.

Code:

import json

d = {'country': 'Germany', 'capital': 'Berlin'}

file = open("dictionary_data.json", "w")

json.dump(d, file)

file.close()

file = open("dictionary_data.json", "r")

output = file.read()

print(output)

file.close()

Output:

Output Console:

{"country": "Germany", "capital": "Berlin"}

Note: The json.dump() method is used to convert a Python object to a JSON string.

Method 3: Using Numpy

Using JSON and Pickle are the best options when it comes to storing a dictionary to a file. But we also have another way of doing so using the Numpy module.

Approach: Call the np.save(filename, dictionary) function to save the file into the disk. To read the data from this file us call the np.load('file.npy', allow_pickle='TRUE').item() function.

Code:

import numpy as np

d = {'country': 'Germany', 'capital': 'Berlin'}

np.save('file.npy', d)

read_d = np.load('file.npy', allow_pickle='TRUE').item()

print(read_d)

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Method 4: Basic Approach

Last but not the least, you can store the dictionary data to a simple text file by simply writing the data to the file using file handling functions.

Approach:

• Call the open('filename.txt', 'w') function to create/open the file in the write mode.
• Use the file.write(str(dictionary)) function to write the dictionary data to the file and then close the file.
• To read the data from this file open up the file and use the file.read() method to read the data from this file.

d = {'country': 'Germany', 'capital': 'Berlin'}

f = open('file.txt', 'w')

f.write(str(d))

f.close()

f = open('file.txt', 'r')

data = f.read()

print(data)

f.close()

Output:

Recommended Reads on File Handling:

• Correct Way to Write line To File in Python
• How to Modify a Text File in Python?

Conclusion

We learned four ways of storing the dictionary data to a file in Python in this article. The most suitable ways of storing the dictionary data in a file are using the JSON or the pickle modules. However, feel free to try out the other ways discussed in this tutorial.

I hope this article helped you. Please subscribe and stay tuned for more interesting tutorials and discussions.

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Problem Formulation and Solution Overview

In this article, you’ll learn how to erase the contents of a file in Python.

To make it more fun, we have the following running scenario:

Let’s say you have a Python script that retrieves the daily stock exchange prices for five (5) Tech Companies and saves it to prices.txt. To ensure no mistakes, you would like to erase the contents of this file before saving the latest data.

Question: How would we write code to erase the contents of a file?

We can accomplish this task by one of the following options:

• Method 1: Use open() and truncate(0)
• Method 2: Use open(), seek(0) and truncate(0)
• Method 3: Use with open()
• Method 4: Use open() and close() on one line
• Bonus: Extract Characters and Erase Remainder

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Preparation

To follow along, copy, paste and save the text below to prices.txt. Move this file to the current working directory.

AAPL,138.22 MMSF,255.67 HPE,14.51 DELL,14.51 MNDT,21.89

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Method 1: Use open() and truncate()

This method erases the contents of a file without removing the file itself using open() and truncate(0).

fp = open('prices.txt', 'w')

fp.truncate(0)

fp.close()

This code opens prices.txt in write mode (w) and saves the output to fp which creates a file object similar to the output below.

<_io.TextIOWrapper name='prices.txt' mode='w' encoding='cp1252'>

Tip: A file object is returned whenever a file is opened in Python. This object allows access to process/manipulate the open file.

Next, fp.truncate(0) is called. This method resizes the said file to a specified number of bytes. If no argument is passed, the current file position is used.

Finally, fp.close() is called to close the open file.

If this code is successful, an empty prices.txt file now resides in the current working directory.

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Method 2: Use open(), seek(0) and truncate(0)

This method opens/re-opens a file and erases the contents without removing the file itself using open(), seek() and truncate(0).

fp = open('prices.txt', 'r+')

fp.seek(0) fp.truncate() 

This code opens/re-opens the prices.txt file in read/write mode (r+) and saves the output to fp which creates a file object similar to the output above.

Next, fp.seek(0) is called to re-position the file pointer (fp) to a given position in the file. In this case, the position is 0 (the top of the file).

Then, fp.truncate(0) is called. This method resizes the said file to a specified number of bytes. If no argument is passed, the current file position is used.

If this code is successful, an empty prices.txt file now resides in the current working directory.

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Method 3: Use with open()

This method erases the contents of a file without deleting the file itself using with open() on one-line!

with open('prices.txt', 'w'): pass

This code calls with open() to open prices.txt for writing (w). Then, the pass statement executes, which does nothing, and the file automatically closes.

Tip: The pass statement is used as a placeholder. When pass executes, nothing happens. This is necessary when code is expected, but no code is required.

If this code is successful, an empty prices.txt file now resides in the current working directory.

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Method 4: Use open() and close() on one line

Also a good option, this method opens a file for writing (open()) and closes said file (close()) using one line of code!

open('prices.txt', 'w').close()

This code uses open() to open prices.txt for writing (w). Since no other code is called, the file contents are erased, and the file closes (close()).

If this code is successful, an empty prices.txt file now resides in the current working directory.

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Bonus: Erase File Contents after Specified Location

What happens if you want to erase everything after the first x number of characters in a file and return the same?

import os fp = os.open('prices.txt', os.O_RDWR|os.O_CREAT)

os.ftruncate(fp, 4)

os.lseek(fp, 0, 0)

str = os.read(fp, 100).decode('utf-8')

print(f"Read String is : {str}")

os.close(fp)

This example could be used for erasing the entire contents of a file. However, let’s retrieve the first four (4) characters from prices.txt (AAPL) and erase the remainder.

First, this code calls in the os library to access the many functions available for interacting with the operating system.

Then, prices.txt is opened in read/write mode, and if the file does not exist, or fails, a new file is created (os.O_RDWR|os.O_CREAT)).

Then, the file is truncated to 4 bytes/characters (os.ftruncate(fp, 4)) and the file pointer (fp) moves to the top of the file (os.lseek(fp, 0, 0)).

Next, the code reads in the first four (4) bytes/characters indicated above and decodes the output (os.read(fp, 100).decode('utf-8')) and saves to str.

The output is sent to the terminal, and the file closes.

Output

Read String is: AAPL

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Summary

These four (4) methods of how to erase the contents of a file should give you enough information to select the best one for your coding requirements.

Good Luck & Happy Coding!

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Programmer Humor

There are only 10 kinds of people in this world: those who know binary and those who don’t.

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There are 10 types of people in the world. Those who understand trinary, those who don’t, and those who mistake it for binary.