make df strategy less complex (#989)

fixes #988

A change in #658 introduced a step to handle str/object dtypes due
to issues handling np.str_. Will need to look into whether that's
still necessary in another PR, but this one compresses a bunch
of `strategy.map` calls to a single one.

This addresses an issue where the strategy would be way too long
for schemas with many str/object columns.

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

pandera/strategies.py

@@ -1103,14 +1103,23 @@ def _dataframe_strategy(draw):
         )
 
         # this is a hack to convert np.str_ data values into native python str.
+        string_columns = []
         for col_name, col_dtype in col_dtypes.items():
             if col_dtype in {"object", "str"} or col_dtype.startswith(
                 "string"
             ):
-                # pylint: disable=cell-var-from-loop,undefined-loop-variable
-                strategy = strategy.map(
-                    lambda df: df.assign(**{col_name: df[col_name].map(str)})
+                string_columns.append(col_name)
+
+        if string_columns:
+            # pylint: disable=cell-var-from-loop,undefined-loop-variable
+            strategy = strategy.map(
+                lambda df: df.assign(
+                    **{
+                        col_name: df[col_name].map(str)
+                        for col_name in string_columns
+                    }
                 )
+            )
 
         strategy = strategy.map(
             lambda df: df if df.empty else df.astype(col_dtypes)