python - OSError unable to create file - invalid argument -

i using python , keras on top of tensorflow train neural networks. when switched ubuntu 16.04 windows 10, model not saved anymore when run following:

filepath = "checkpoint-"+str(f)+model_type+"-"+optimizer_name+"-{epoch:02d}-{loss:.3f}.hdf5" checkpoint = modelcheckpoint(filepath, monitor='loss', verbose=1, save_best_only=true, mode='min') callbacks_list = [checkpoint] 

and later on:

model.fit(x, y,       batch_size=128,       epochs=1,       shuffle=false,       callbacks=callbacks_list) 

i error:

oserror: unable create file (unable open file: name = 'checkpoint-<_io.textiowrapper name='data/swing-projects100-raw/many-chunks/log-gamma-f3.txt' mode='a' encoding='cp1252'>2l128-adam-0.001-{epoch:02d}-{loss:.3f}.h5', errno = 22, error message = 'invalid argument', flags = 13, o_flags = 302)

i have keras 2.0.8 , h5py 2.7.0 installed via conda.

i tried

filepath = "checkpoint-"+str(f)+model_type+"-"+optimizer_name+"-{epoch:02d}-{loss:.3f}.hdf5"  open(filepath, "w") f:   f.write("test.") 

and got similar error:

oserror: [errno 22] invalid argument: "checkpoint-<_io.textiowrapper name='data/swing-projects100-raw/many-chunks/log-gamma-f3.txt' mode='a' encoding='cp1252'>2l128-adam-0.001-{epoch:02d}-{loss:.3f}.hdf5"

when removed str(f) filepath, worked. f integer , don't know why caused error, removing string solved problem.

let me know if know why.