Concepts / Understanding File Modes in Python

Understanding File Modes in Python

Binary files require byte-for-byte copying from a remote URL to your local disk.

  • Programming

From URL to Local File

When a program downloads an image, video, or another non-text file, the goal is a byte-for-byte transfer from the remote URL to local disk. Python can retrieve the complete response with urllib.request.urlopen(url).read(), keep that downloaded data in memory, and then write it to a local file. The file must be opened in binary write mode so the downloaded binary data is preserved.

Remote URLbinary fileurllib responsedownloaded dataPython memorybinary dataLocal filebyte-for-byte copyrequestread()write(data)
How does binary data move from a remote URL through Python memory and into a local file without being changed?

The Download-and-Write Pattern

The pattern has two main operations. First, urllib.request.urlopen(url).read() retrieves the entire file and stores the result in memory. Second, open(filename, 'wb').write(data) opens a local file in binary write mode and writes the downloaded data. The same data moves through the program in this order: remote file, Python memory, and local file.

Open URLurlopen(url)Read entire fileread()File in memorycomplete binary dataWrite local filewbresponsestores all datapasses data
What happens to memory when urllib downloads the complete file before the program writes it to disk?

Text Mode and Binary Mode

File mode tells Python how the local file will be handled. For this download pattern, the data is binary data, so the local file must be opened with wb. The w mode is text mode, and using it for binary data can corrupt that data. The b in wb identifies binary mode, while w identifies writing.

Text modewBinary modewbText datanot for binary copyBinary databyte-for-byte copywriteswrites
What is different about the data Python reads and writes in text mode compared with binary mode?
ModePurpose in this topicResult for a binary download
wText write modeCan corrupt binary data
wbBinary write modeEssential for byte-for-byte copying

The important distinction is whether the local file is opened for text writing or binary writing.

A Complete Download Trace

Python programRemote URLbinary fileLocal diskdestination fileurlopen(url)read() dataopen(filename, wb).write(data)
What are the steps from requesting a URL to receiving the response data and saving it locally?
Code walkthrough
import urllib.request

url = "https://example.com/file"
data = urllib.request.urlopen(url).read()
open("downloaded_file", "wb").write(data)
  1. 1
    The first line makes urllib.request available.
  2. 2
    The url variable identifies the remote file location.
  3. 3
    The read() call retrieves the entire file and stores the downloaded data in data.
  4. 4
    The final line uses wb, not w, so the binary data is written as a byte-for-byte local copy.
  • Imports urllib.request so the program can use urlopen.
  • Names the remote location with url.
  • Calls urlopen(url).read() to download the entire file into data.
  • Opens downloaded_file in binary write mode and writes data to it.
Output
The local file contains the downloaded binary data.

Why wb Matters

openopens a filefilenamedestination pathwbbinary write modewrite(data)stores downloaded datareceivespaired withenables
How does opening a local file with wb allow Python to write the downloaded bytes unchanged?

In open(filename, 'wb'), filename identifies the local destination and wb selects binary write mode. The following write(data) operation places the downloaded data into that file. Replacing wb with w changes the mode to text writing, which is not suitable for this binary-copy task and can corrupt the data.

What do you think happens?

What should replace the mode string when the downloaded file is an image or video?

  • w

  • wb

Reveal answer
Answer:

wb

The source pattern uses binary write mode because binary files require byte-for-byte copying. Text mode w can corrupt binary data.

Common Download Mistakes

  • Common mistake

    Opening the destination with w instead of wb.

    What it looks like
    open(filename, 'w').write(data)
    Why it fails

    Text mode is not appropriate for binary data and can corrupt the downloaded file.

    Do this instead
    Use open(filename, 'wb').write(data).
  • Common mistake

    Forgetting that read() downloads the entire file before the write occurs.

    What it looks like
    data = urllib.request.urlopen(url).read()
    Why it fails

    The complete file is stored in memory, so the pattern requires enough available RAM for the file.

    Do this instead

    Use this simple pattern for files smaller than the available RAM.

  • Common mistake

    Treating the remote response and local save as one unexplained operation.

    What it looks like

    Skipping the distinction between read() and write(data)

    Why it fails

    The pattern has two stages: retrieving the complete data into memory and writing that data to disk.

    Do this instead

    Trace the data from urlopen(url).read() into the variable and then into open(filename, 'wb').write(data).

Practice the Trace

EASY

A program downloads a non-text file with urllib.request.urlopen(url).read() and stores the result in data. Complete the destination operation so the downloaded binary data is written correctly to a local file.

Hints
  • The destination must use binary write mode.
  • The source pattern writes data after opening the local filename.
  • The required mode is wb.
python
Worked example

Tracing the Complete File

Identify where the complete binary file exists at each stage of the download pattern.

  1. 1
    Request

    urllib.request.urlopen(url) accesses the remote URL.

  2. 2
    Retrieve

    Calling read() retrieves the entire file and places the downloaded data in memory through the variable data.

  3. 3
    Save

    Opening the destination with wb and calling write(data) writes the binary data to the local file.

  4. 4
    Check memory

    Because the entire file was read before writing, the file must be smaller than the available RAM for this simple pattern to be suitable.

Result

The correct flow is remote URL to complete data in memory to local file opened with wb.

Key Takeaways

  1. 1
    Use urllib.request.urlopen(url).read() to retrieve an entire binary file into memory.
  2. 2
    Use open(filename, 'wb').write(data) to write the downloaded data to a local file.
  3. 3
    Binary write mode wb is essential for byte-for-byte copying; text mode w can corrupt binary data.
  4. 4
    The complete-download pattern is suitable for files smaller than the available RAM.
  5. 5
    Always trace the data through the three stages: remote URL, Python memory, and local disk.

At a glance

  • A binary download should preserve the file byte for byte from the remote URL to local disk.
  • urllib.request.urlopen(url).read() downloads the entire file into memory.
  • open(filename, 'wb').write(data) writes that binary data to a local file.
  • Using text mode w for binary data can corrupt the file.
  • Reading the entire file at once requires enough available RAM.

Related Topics

Related concepts

More concepts starting with U