Why Does tqdm Print to sys.stderr? A Deep Dive into Progress Bars
Have you ever noticed how tqdm's progress bars mysteriously appear below your regular output in the terminal? It might seem confusing, but it's intentional! Understanding why tqdm chooses sys.stderr instead of sys.stdout requires delving into the intricacies of how progress bars interact with your code and your terminal.
The Scenario:
Imagine you're running a script that processes a large dataset, and you've decided to use tqdm to visualize the progress. You might write code similar to this:
import time
from tqdm import tqdm
for i in tqdm(range(100)):
time.sleep(0.01)
print(f"Processing item {i}")
Running this code reveals a peculiar behavior: the progress bar displays below your "Processing item..." messages.
The Explanation:
The root of this behavior lies in the distinction between sys.stdout and sys.stderr:
sys.stdout: This is your standard output stream, where your program's main output is displayed. Think of it as the "main stage" for your code's messages.sys.stderr: This is the standard error stream, intended for error messages and warnings. Imagine it as a "side stage" where important notifications are delivered.
tqdm's Choice:
tqdm uses sys.stderr to avoid interfering with your regular program output. Imagine this scenario:
- Your program is displaying important data in real-time.
- A progress bar, printed to
sys.stdout, might overwrite your data, obscuring it from view.
By utilizing sys.stderr, tqdm ensures that its progress bar remains visible without disrupting the flow of your program's primary output.
Key Benefits:
- Clarity: Progress bars are separated from regular output, making it easier to follow both.
- Flexibility: You can redirect or filter
sys.stderrseparately, giving you greater control over the display of progress information.
Going Further:
You can easily adjust the output location if desired. For example:
from tqdm import tqdm
with tqdm(total=100, file=sys.stdout) as pbar:
for i in range(100):
# ... your code ...
pbar.update(1)
In this modified code, tqdm now writes to sys.stdout and the progress bar appears alongside your regular output.
In Conclusion:
While it might seem unusual at first, tqdm's choice to print to sys.stderr is a thoughtful design decision, aiming to enhance the readability and usability of progress bars within your code. By understanding the reasons behind this behavior, you can better control and leverage the powerful features of tqdm for efficient and informative progress visualization.
