OSError unable to create file - invalid argument

Question

I am using Python and Keras on top of Tensorflow to train my neural networks. When I switched from Ubuntu 16.04 to Windows 10, my model could not be saved anymore when I run the 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 get this Error:

OSError: Unable to create file (Unable to 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 and h5py 2.7.0 installed via conda.

I tried

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

with open(filepath, "w") as f:
  f.write("Test.")

and got a 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"

Solution

When I removed str(f) from the filepath, it worked. f is an Integer and I don't know why it caused the error, but removing it from the string solved my problem.

Let me know if you know exactly why.