The Challenge of Checking File Size Before Downloading with Python

When programming in Python, particularly when dealing with file downloads, it can be quite frustrating to determine the size of files before starting the downloading process. This situation often arises when you want to compare the server’s file size with a local version to check if an update is available. In this blog post, we will explore how to retrieve the file size from the server using Python’s urllib library and address common issues that may arise during this process.

Understanding the Problem

Suppose you are downloading files from a web server, such as .TXT or .ZIP files. You notice that while the download completes successfully, you can’t determine if the file has been updated on the server unless you download it. Ideally, you would like to know the file size beforehand to make a comparison. The various methods of downloading and handling files can complicate this task, especially with issues like line ending conversions that can lead to size discrepancies.

Solution: Retrieve the File Size Before Downloading

In order to get the size of a file before downloading it, follow these steps using the urllib library to make a request and extract the file size.

Step 1: Import Required Libraries

We will need to import the urllib and os libraries to handle HTTP requests and interact with the file system.

import urllib
import os

Step 2: Open the File URL

The first step is opening the URL from which you want to download the file.

link = "http://www.someurl.com/myfile.txt"
site = urllib.urlopen(link)

Step 3: Retrieve Metadata

Once the site is opened, you can retrieve the metadata that includes the file size (Content-Length) using the info() method.

meta = site.info()
file_size = int(meta.getheaders("Content-Length")[0])
print(f"Content-Length: {file_size}")

This will give you the size of the file on the server which you can store in a variable for future comparison.

Step 4: Check Local File Size

Before downloading, you should also check the size of the local file (if it exists). This can be done using the os module.

if os.path.isfile("myfile.txt"):
    local_size = os.stat("myfile.txt").st_size
    print(f"Local file size: {local_size}")
else:
    local_size = 0

Step 5: Compare and Download

Now that you have both sizes, you can compare them to decide if you need to download the updated file.

if file_size != local_size:
    print("Downloading the file...")
    with open("myfile.txt", "wb") as f:
        f.write(site.read())
else:
    print("No download needed, the file is up-to-date.")

Step 6: Closing the Connection

Don’t forget to close the website connection after your work is done.

site.close()

Final Code Example

Here’s the complete code with all the steps integrated:

import urllib
import os

link = "http://www.someurl.com/myfile.txt"
site = urllib.urlopen(link)
meta = site.info()
file_size = int(meta.getheaders("Content-Length")[0])
print(f"Content-Length: {file_size}")

if os.path.isfile("myfile.txt"):
    local_size = os.stat("myfile.txt").st_size
    print(f"Local file size: {local_size}")
else:
    local_size = 0

if file_size != local_size:
    print("Downloading the file...")
    with open("myfile.txt", "wb") as f:
        f.write(site.read())
else:
    print("No download needed, the file is up-to-date.")

site.close()

Common Issues: The Binary Mode Confusion

A notable point to consider is that when reading and writing files, always open your file streams in binary mode ('rb' for reading and 'wb' for writing). This commonly resolves size discrepancies due to line ending conversions, especially when downloading files that contain text. Here’s how to ensure you’re working in binary mode:

# Open for binary write
open(filename, "wb") 

# Open for binary read
open(filename, "rb") 

Conclusion

In this post, we explored how to check the file size on a server before downloading it in Python. This is useful for updating files intelligently and prevents unnecessary downloads. With the provided steps and code samples, you should be well-equipped to implement this functionality in your own Python applications.