Reading Text Files with urllib
Binary files require byte-for-byte copying from a remote URL to your local disk.
From Remote Bytes to Local Storage
A URL can point to a non-text file such as an image or video. To copy that file to your computer, the basic task is byte-for-byte transfer: retrieve the file data from the remote URL, keep that data in memory, and write the same data to a local file.
The Download-and-Write Pattern
The operation has two main stages. First, urllib.request.urlopen(url).read() downloads the entire remote file into memory. The resulting data is then written to a local file with open(filename, 'wb').write(data). The local file must be opened in binary write mode so that the downloaded binary data is copied byte for byte.
from urllib.request import urlopen url = "https://example.com/file.bin" data = urlopen(url).read() open("file.bin", "wb").write(data)
Why wb Matters
The mode wb means binary write mode. It allows the local file to receive the downloaded binary data. Text write mode, w, is not suitable because it can corrupt binary data instead of preserving the original bytes.
Memory Before Disk
Calling read() without a size downloads the entire file into memory before the write operation begins. This simple pattern is appropriate for files smaller than the available RAM. As the downloaded file becomes larger, the amount of memory needed to hold data also becomes larger.
Use the read-all-at-once pattern when the file is smaller than the available RAM. Before applying it to a large binary file, consider that the complete downloaded file must be held in memory before it is written locally.
Mistakes in Binary Downloads
Opening the destination with w instead of wb.
Text mode can corrupt binary data.
Fix:
Open the destination with open(filename, "wb").Assuming read() writes directly to disk.
The read operation downloads the entire file into memory; a separate write operation is needed.
Fix:
Write the downloaded data to the local file with binary write mode.Ignoring the size of the file being downloaded.
The complete file is loaded into memory before writing.
Fix:
Apply this simple pattern to files smaller than the available RAM.
Practice the Transfer
Write the two Python statements needed to download a small binary file from a URL into memory and then save it as local.bin. Use the variable name url for the URL and data for the downloaded content.
Hints
- Use urlopen(url).read() to retrieve the entire file.
- Open the destination with binary write mode, wb.
- Write data after opening the local file.
A Complete Small-File Transfer
Copy a non-text file from a URL to a local file using the read-all-at-once pattern.
Retrieve: Call urlopen(url).read() so the complete remote file is stored in data.
Open: Open the local destination with wb because the data is binary.
Write: Write data to the local destination to preserve the downloaded bytes.
Check the constraint: Confirm that the file is smaller than the available RAM before using this pattern.
The pattern retrieves the entire binary file into memory and writes it to a local file in binary mode.
Key Takeaways
- Use urllib.request.urlopen(url).read() to download an entire remote file into memory.
- Write the downloaded data with open(filename, 'wb').write(data).
- Use wb rather than w because text mode can corrupt binary data.
- The read-all-at-once pattern requires enough available RAM for the complete file.
- The process is a byte-for-byte copy from the remote URL to local storage.