Add optional show_epoch flag to ProgressBar for customizable epoch display

[SpandanBhoiIITM]

Closes #3659

Description

This PR adds a show_epoch parameter to ignite.handlers.tqdm_logger.ProgressBar.

Currently, epoch information (e.g. "[5/10]") is automatically appended to the
progress bar description when max_epochs > 1. This behavior makes it
difficult to align multiple progress bars (e.g. trainer and evaluator) with
custom formatting.

The new show_epoch flag (default=True) allows users to disable automatic
epoch display while preserving backward compatibility.

All existing behavior remains unchanged by default.

Checklist

• New tests are added (can be added if required)
• New docstrings are updated
• Documentation update required

[TahaZahid05]

@SpandanBhoiIITM Thanks for the contribution.

1. Could you please ensure your PR aligns with the code style, documentation, and test requirements mentioned in our CONTRIBUTING.md. This includes fixing the indentation (running black or pre-commit), adding docstrings, and including a unit test. Also, please replace {issue_number} in the description with the actual issue ID by opening an issue.
2. Instead of a specific show_epoch boolean, would it be possible to make this more general? We could use a description_format parameter (defaulting to "{desc} [{epoch}/{max_epochs}]"). This would allow users to remove the epoch by setting it to "{desc}", while also supporting other custom formats without needing more boolean flags in the future.

[vfdev-5]

@SpandanBhoiIITM @TahaZahid05 I suggest to discuss the problem before reviewing the code change. We should not introduce alternative API if already exists a solution to the problem.

[TahaZahid05]

@vfdev-5.
Right now, the only way a user can hide the epoch is by creating a completely custom bar_format string and deleting the {desc} placeholder from it entirely.

However, doing this is flawed because it breaks the progress bar's ability to have a title. If a user removes the {desc} placeholder, tqdm completely ignores the desc="..." argument. Users are forced to permanently type their title directly into the formatting string itself (like bar_format="Training | {bar}... "). If they do this, standard features ProgressBar(desc="Training") will silently stop working, and they can no longer update the title while the script is running.

@SpandanBhoiIITM Given this, rather than introducing a strict show_epoch boolean, I believe we should think about a description_format string parameter.

For instance, Users can pass description_format="{desc}" to drop the epoch string. This gives us customizability without needing more boolean flags down the line.

What do you think?

[vfdev-5]

Currently, epoch information (e.g. "[5/10]") is automatically appended to the
progress bar description when max_epochs > 1. This behavior makes it
difficult to align multiple progress bars (e.g. trainer and evaluator) with
custom formatting.

@SpandanBhoiIITM can provide a small reproducer of what you are trying to achieve to see explicitly what does not work as desired. Thanks!

For example, this code:

from ignite.engine import Engine, Events
from ignite.handlers import ProgressBar


train_data = range(100)
eval_data = range(40)
max_epochs = 10


def train_step(engine, batch):
    import time

    time.sleep(0.01)
    return None

trainer = Engine(train_step)
evaluator = Engine(lambda e, b: None)


@trainer.on(Events.EPOCH_COMPLETED(every=2))
def run_validation():
    evaluator.run(eval_data)

ProgressBar(persist=True, desc="Training").attach(trainer)
ProgressBar(persist=True, desc="Validation").attach(evaluator)

trainer.run(train_data, max_epochs=max_epochs)

gives the following representation in Jupyter notebook

and if we are bothered that progress bars are not aligned we can redefine the bar format as we would like:

bformat = "{percentage:3.0f}%|{bar}{postfix} [{elapsed}<{remaining}] - {desc}[{n_fmt}/{total_fmt}]"
ProgressBar(persist=True, bar_format=bformat, desc="Training").attach(trainer)
ProgressBar(persist=True, bar_format=bformat, desc="Validation").attach(evaluator)

[SpandanBhoiIITM]

@vfdev-5 @TahaZahid05 Thanks for the feedback!

My motivation for adding show_epoch was to make it easier to hide the epoch
information while still keeping the desc argument usable.

Right now the only way to remove the epoch is by rewriting the whole
bar_format. But if {desc} is removed from bar_format, tqdm ignores the
desc argument entirely, so the title can't be controlled dynamically.

The show_epoch flag was meant as a simple way to disable the epoch display
without rewriting the full format string.

I, however agree that a description_format parameter could be a more
flexible solution than a boolean flag. I'm happy to update the implementation
in that direction if you think that would be better.

[TahaZahid05]

@SpandanBhoiIITM First, can you kindly reproduce some code examples of progress bar formats that currently do not work as desired? This would help us in understanding where the current implementation is lacking. You can refer to the conversation above for more guidance.

[SpandanBhoiIITM]

I ran a small example to illustrate the current behavior and the motivation behind this change.

from ignite.engine import Engine, Events
from ignite.handlers import ProgressBar

train_data = range(100)
eval_data = range(40)
max_epochs = 4

def train_step(engine, batch):
    import time
    time.sleep(0.01)

trainer = Engine(train_step)
evaluator = Engine(lambda e, b: None)

@trainer.on(Events.EPOCH_COMPLETED(every=2))
def run_validation():
    evaluator.run(eval_data)

ProgressBar(persist=True, desc="Training").attach(trainer)
ProgressBar(persist=True, desc="Validation").attach(evaluator)

trainer.run(train_data, max_epochs=max_epochs)

---

Current behavior

This produces the following representation in the terminal / Jupyter notebook:

Here the epoch information [epoch/max_epochs] is automatically appended to the progress bar description, for example:

Training [1/4]
Training [2/4]
Validation [40/40]

---

Attempt using bar_format

As @vfdev-5 mentions , It is possible to modify the layout using bar_format, for example:

bformat = "{percentage:3.0f}%|{bar}{postfix} [{elapsed}<{remaining}] - {desc}[{n_fmt}/{total_fmt}]"

ProgressBar(persist=True, bar_format=bformat, desc="Training").attach(trainer)
ProgressBar(persist=True, bar_format=bformat, desc="Validation").attach(evaluator)

However, if a user only wants to remove the epoch display, they currently need to redefine the entire bar_format.

---

With the proposed change

Using the new parameter:

from ignite.engine import Engine, Events
from ignite.handlers import ProgressBar
import time

data = range(20)

def train_step(engine, batch):
    time.sleep(0.05)
    return batch


trainer = Engine(train_step)


ProgressBar(
    persist=True,
    desc="Training",
    show_epoch=False   
).attach(trainer)


trainer.run(data, max_epochs=3)

Here the [epoch/max_epochs] part is removed while still keeping the dynamic description provided through desc.

This allows users to hide the epoch indicator without rewriting the entire bar_format, which makes the API simpler for this use case.

[vfdev-5]

@SpandanBhoiIITM what's the point to remove the appended epoch/max-epochs info ?

To move forward with this, I suggest to create an issue discussing this feature. If there's a support of the community on it, we can consider this PR. For now, personally I do not see the usefulness of this feature.

[SpandanBhoiIITM]

@vfdev-5 Thanks for the suggestion. I have opened an issue to discuss the feature here: #3659.