Expected behavior and actual behavior.

Expected behavior: installing rasterio from pip on a fresh Python environment, and trying to open a local file with it. This should succeed without needing additional requirements.

Actual behavior: rio info a.tiff on a local a.tiff file, for example, gives

Traceback (most recent call last):
  File "/Users/guillaumelostis/.venv/rio/bin/rio", line 10, in <module>
    sys.exit(main_group())
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/click/core.py", line 1134, in invoke
    Command.invoke(self, ctx)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/rasterio/rio/main.py", line 103, in main_group
    ctx.obj["env"] = rasterio.Env(**envopts)
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/rasterio/env.py", line 195, in __init__
    self.session = AWSSession()
  File "/Users/guillaumelostis/.venv/rio/lib/python3.7/site-packages/rasterio/session.py", line 198, in __init__
    import boto3
ModuleNotFoundError: No module named 'boto3'

Steps to reproduce the problem.

pip install rasterio on a fresh Python environment. This installs rasterio==1.0.24 using the Mac OS wheels on my machine.

Then running rio info a.tiff on a local file gives the traceback above. This is due to the fact that I have the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables set on my machine since I occasionally work with files from S3.
This causes this condition: https://github.com/mapbox/rasterio/blob/4aefb4674d19f29cce8a408819aa001151b204d1/rasterio/env.py#L194-L195 to be true, and therefore an AWSSession is used to open my local file.

I am aware that I can easily solve my problem by running pip install "rasterio[s3]", or by removing the environment variables when working with local files, but both these solutions require extra work and could mislead new users. I know how to work around this issue, but I am just wondering of you agree that this is a bug, or if you think that this is the expected behavior, in which case I don't mind you closing the issue.

I have also found out that opening a local file using:

import rasterio
f = rasterio.open("a.tiff")

works, while this does not:

import rasterio
with rasterio.open("a.tiff") as f:
    pass

This is logical since the second solution uses rasterio.env while the first one doesn't.

I don't have a good knowledge of the inner workings of rasterio, but falling back to a basic session if boto3 requirement is not met could maybe be a way to workaround the problem I am facing?

Operating system

All

Rasterio version and provenance

The 1.0.24 Mac OS wheel installed from PyPI using pip 19.0.3.