`librosa.load()` silently ignores the `duration` parameter if it is a string

Description

When using soundfile, librosa.load() silently ignores the duration parameter if it is a string. I haven’t tested other input types, but passing "4" as the duration should either work as expected or raise an exception rather than silently being ignored,

Steps/Code to Reproduce

1. Ensure you’re loading a format supported by soundfile (eg. WAV or FLAC)
2. Pass a string as the duration parameter of librosa.load():

audio, sr = librosa.load('file.flac', duration="4")  # Note that `duration` is a string
print((len(audio) / sr) == 4)

This generally comes out False since the duration parameter is ignored and the whole file is loaded.

Expected Results

The duration of the loaded audio should be 4 seconds (if the string is castable to an int), OR a TypeError (or some other exception) should be raised.

Versions

>>> import platform; print(platform.platform())
Windows-10-10.0.18362-SP0
>>> import sys; print("Python", sys.version)
Python 3.7.3 (default, Apr 24 2019, 15:29:51) [MSC v.1915 64 bit (AMD64)]
>>> import numpy; print("NumPy", numpy.__version__)
NumPy 1.16.4
>>> import scipy; print("SciPy", scipy.__version__)
SciPy 1.3.1
>>> import librosa; print("librosa", librosa.__version__)
librosa 0.7.0

[konoikon]

I do not believe that it is being ignored. In fact, there is something more interesting going on. Here’s a part of load():

if duration is not None:
	frame_duration = int(duration * sr_native)

When duration="4" is passed, the string is multiplied and results in a frame duration of a sr_native number of 4s. Does that seem right?

[lostanlen]

@konoikon i’m not a big fan of try-catch statements unless absolutely necessary. They are difficult to specify, difficult to test, and difficult to maintain. I am also wary of implicit type conversions at runtime. I would prefer a simple if statement on the type and value and duration with a clear error message, thereby deferring the responsibility of supplying the correct type to the user.