Add case_when & if_else expression helpers

Created expressions.py and moved all the expressions
related functions and class from utils.py to there.

doc/api.rst

@@ -100,6 +100,23 @@ Two table verbs
    right_join
    semi_join
 
+
+Expression helpers
+==================
+These classes can be used construct complicated conditional assignment
+expressions.
+
+.. currentmodule:: plydata.expressions
+
+.. autosummary::
+   :toctree: generated/
+   :nosignatures:
+   :template: main.rst
+
+   case_when
+   if_else
+
+
 Options
 =======
 
@@ -126,10 +143,4 @@ Useful Functions
    :template: main.rst
 
    Q
-
-n()
----
-This is a special internal function that returns the number of
-rows in the current groups. It can be used in verbs like
-:class:`~plydata.verbs.summarize`, :class:`~plydata.verbs.define`.
-and :class:`~plydata.verbs.create`.
+   n

plydata/__init__.py

@@ -1,6 +1,7 @@
 from .one_table_verbs import *  # noqa
 from .two_table_verbs import *  # noqa
 from .helper_verbs import *     # noqa
+from .expressions import *      # noqa
 from ._version import get_versions
 
 __version__ = get_versions()['version']

plydata/dataframe/common.py

@@ -9,8 +9,9 @@
 import pandas.api.types as pdtypes
 import numpy as np
 
+from ..expressions import Expression
 from ..types import GroupedDataFrame
-from ..utils import get_empty_env, unique, Expression
+from ..utils import get_empty_env, unique
 
 
 def _get_groups(verb):
@@ -458,9 +459,9 @@ def get(cls, verb):
             return cls._if(verb)
 
 
-def make_expressions(verb):
+def build_expressions(verb):
     """
-    Make expressions
+    Build expressions for helper verbs
 
     Parameters
     ----------

plydata/dataframe/helpers.py

@@ -7,8 +7,8 @@
 import numpy as np
 
 from ..types import GroupedDataFrame
-from ..utils import Expression
-from .common import _get_groups, Selector, make_expressions
+from ..expressions import Expression
+from .common import _get_groups, Selector, build_expressions
 from .one_table import arrange, create, define, group_by
 from .one_table import mutate, rename, summarize
 
@@ -171,7 +171,7 @@ def _make_verb_helper(verb_func, add_groups=False):
 
     @wraps(verb_func)
     def _verb_func(verb):
-        verb.expressions, new_columns = make_expressions(verb)
+        verb.expressions, new_columns = build_expressions(verb)
         if add_groups:
             verb.groups = new_columns
         return verb_func(verb)