Make module loading lazy (#162)

* Make module loading lazy

* Catch less in JuliaModule.__getattr__

* Support star import

* Special handling for Main module

It adds an easier way to set variables in Julia's global namespace:

>>> from julia import Main
>>> Main.xs = [1, 2, 3]

* Deprecate Julia.__getattr__ (and make it lazy)

fixes #144

* Initialize Julia in JuliaModuleLoader if required

This makes `from julia import <Julia module>` works without the initial
setup.  Note that a custom setup can still be done by calling
`julia.Julia` with appropriate arguments *before* trying to import Julia
modules.

closes #39, #79

* More interfaces to JuliaMainModule

* Document the new API

* Properly remove prefix

* Fix JuliaModule.__all__; add .__dir__

* Cache original os.environ for tests

* Simplify JuliaModule.__try_getattr

* Add LegacyJulia to break infinite recursion

* Don't import in isamodule

Otherwise, trying to access undefined variables such as
`julia.Base.__path__` produces a warning from Julia before
`JuliaModule.__getattr__` raising `AttributeError`.

* Remove core._orig_env; it can be done in test code

* Use Enums as example sub-module

as Base.REPL was pulled out from Base in Julia 0.7.

* More import tests

* Remove noop code

* Fix wording

README.md

@@ -58,24 +58,72 @@ If you run into problems using `pyjulia`, first check the version of `PyCall.jl`
 
 Usage
 -----
-To call Julia functions from python, first import the library
+
+`pyjulia` provides a high-level interface which assumes a "normal"
+setup (e.g., `julia` is in your `PATH`) and a low-level interface
+which can be used in a customized setup.
+
+### High-level interface
+
+To call a Julia function in a Julia module, import the Julia module
+(say `Base`) with:
+
+```python
+from julia import Base
+```
+
+and then call Julia functions in `Base` from python, e.g.,
 
 ```python
-import julia
+Base.sind(90)
 ```
 
-then create a Julia object that makes a bridge to the Julia interpreter (assuming that `julia` is in your `PATH`)
+Other variants of Python import syntax also work:
 
 ```python
-j = julia.Julia()
+import julia.Base
+from julia.Base import LinAlg   # import a submodule
+from julia.Base import sin      # import a function from a module
 ```
 
-You can then call Julia functions from python, e.g.
+The global namespace of Julia's interpreter can be accessed via a
+special module `julia.Main`:
 
 ```python
-j.sind(90)
+from julia import Main
 ```
 
+You can set names in this module to send Python values to Julia:
+
+```python
+Main.xs = [1, 2, 3]
+```
+
+which allows it to be accessed directly from Julia code, e.g., it can
+be evaluated at Julia side using Julia syntax:
+
+```python
+Main.eval("sin.(xs)")
+```
+
+### Low-level interface
+
+If you need a custom setup for `pyjulia`, it must be done *before*
+importing any Julia modules.  For example, to use the Julia
+interpreter at `PATH/TO/MY/CUSTOM/julia`, run:
+
+```python
+from julia import Julia
+j = julia.Julia(jl_runtime_path="PATH/TO/MY/CUSTOM/julia")
+```
+
+You can then use, e.g.,
+
+```python
+from julia import Base
+```
+
+
 How it works
 ------------
 PyJulia loads the `libjulia` library and executes the statements therein.

julia/__init__.py

@@ -1 +1 @@
-from .core import Julia, JuliaError
+from .core import JuliaError, LegacyJulia as Julia