PyTorch: How to change the learning rate of an optimizer at any given moment (no LR schedule)

So the learning rate is stored in optim.param_groups[i]['lr']. optim.param_groups is a list of the different weight groups which can have different learning rates. Thus, simply doing:

for g in optim.param_groups:
    g['lr'] = 0.001

will do the trick.

Alternatively,

as mentionned in the comments, if your learning rate only depends on the epoch number, you can use a learning rate scheduler.

For example (modified example from the doc):

torch.optim.lr_scheduler import LambdaLR
optimizer = torch.optim.SGD(model.parameters(), lr=0.1, momentum=0.9)
# Assuming optimizer has two groups.
lambda_group1 = lambda epoch: epoch // 30
lambda_group2 = lambda epoch: 0.95 ** epoch
scheduler = LambdaLR(optimizer, lr_lambda=[lambda1, lambda2])
for epoch in range(100):
    train(...)
    validate(...)
    scheduler.step()

Also, there is a prebuilt learning rate scheduler to reduce on plateaus.

Instead of a loop in @patapouf_ai's answer, you can do it directly via:

optim.param_groups[0]['lr'] = 0.001

Cheers