How to use Bazel's py_library imports argument

I think you're on the right track, but you're running into a subtle problem due to the ordering of the python sys.path.

If I run your example and print out sys.path in example.py, I see that the path contains in order:

bazel-out/local-fastbuild/bin/example.runfiles
bazel-out/local-fastbuild/bin/example.runfiles/boto3/boto3
bazel-out/local-fastbuild/bin/example.runfiles/boto3

The second line is due to the imports = ['boto3'] in your WORKSPACE file.

I think you want the third line to be where you get import boto3 from, because you want python to see bazel-out/local-fastbuild/bin/example.runfiles/boto3/boto3/__init__.py.

So when python evaluates import boto3, it sees bazel-out/local-fastbuild/bin/example.runfiles/boto3/__init__.py from the first entry and uses that, instead of bazel-out/local-fastbuild/bin/example.runfiles/boto3/boto3/__init__.py from the third entry.

I think the answer here, is to name your "workspace" something other than the directory it contains. For example:

# WORKSPACE
new_git_repository(
  name = "boto3_archive",
  commit = "8227503d7b1322b45052a16b197ac41fedd634e9", # 1.4.4
  remote = "https://github.com/boto/boto3.git",
  build_file_content = _BOTO3_BUILD_FILE,
)

# BUILD
py_binary(
  name = "example",
  srcs = [ "example.py" ],
  deps = [
    "@boto3_archive//:boto3",
  ],
)

When I do this in your example, I get the following error: ImportError: No module named dateutil.parser, which I think is progress.

Files

$ tree
.
├── BUILD
├── WORKSPACE
├── requirements.txt
└── src
    └── main.py

requirements.txt

$ cat requirements.txt 
boto3==1.13.4

WORKSPACE

$ cat WORKSPACE 
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "rules_python",
    url = "https://github.com/bazelbuild/rules_python/releases/download/0.0.2/rules_python-0.0.2.tar.gz",
    strip_prefix = "rules_python-0.0.2",
    sha256 = "b5668cde8bb6e3515057ef465a35ad712214962f0b3a314e551204266c7be90c",
)
load("@rules_python//python:repositories.bzl", "py_repositories")

py_repositories()

# Only needed if using the packaging rules.
load("@rules_python//python:pip.bzl", "pip_repositories")

pip_repositories()


# Python external packages installation
load(
    "@rules_python//python:pip.bzl", "pip3_import"
)

pip3_import(
    name = "lambda_deps",
    requirements = "//:requirements.txt", # Top level requirements.txt file
)

load("@lambda_deps//:requirements.bzl", "pip_install")
pip_install()

BUILD

$ cat BUILD
load(
    "@lambda_deps//:requirements.bzl",
    "requirement"
)

py_binary(
    name = 's3_test',
    main = 'main.py',
    srcs = ['src/main.py'],
    deps = [
        requirement('boto3')
    ]
)

src/main.py

$ cat src/main.py
import boto3


def hello_boto3():
    print('hello', boto3.client('s3'))


if __name__ == '__main__':
    hello_boto3()

Build and run

$ bazel clean && bazel build //:s3_test
Extracting Bazel installation...
Starting local Bazel server and connecting to it...
INFO: Starting clean (this may take a while). Consider using --async if the clean takes more than several minutes.
INFO: Analyzed target //:s3_test (24 packages loaded, 1234 targets configured).
INFO: Found 1 target...
Target //:s3_test up-to-date:
  bazel-bin/s3_test
INFO: Elapsed time: 11.666s, Critical Path: 0.25s
INFO: 0 processes.
INFO: Build completed successfully, 5 total actions

$ ./bazel-bin/s3_test
hello <botocore.client.S3 object at 0x7ff1b6686b38>