For things with a fixed length, it looks it up ahead of time (i.e. things that return things when you call len() on them). For other things such as generators,

For things with a fixed length, it looks it up ahead of time (i.e. things that return things when you call len() on them). For other things such as generators, you can supply a total=N value when you create the progress bar. If you dont know the total ahead of time, it doesnt give you a percent completion, but still tells you thinks like number of iterations per second and number of iterations finished.