Read specific columns as str

src/starfile/functions.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Dict, List, Union
+from typing import TYPE_CHECKING, Dict, List, Union, Optional
 
 if TYPE_CHECKING:
     import pandas as pd
@@ -15,15 +15,32 @@
     from os import PathLike
 
 
-def read(filename: PathLike, read_n_blocks: int = None, always_dict: bool = False):
-    """
-    Read a star file into a pandas dataframe or dict of pandas dataframes
+def read(
+    filename: PathLike,
+    read_n_blocks: Optional[int] = None,
+    always_dict: bool = False,
+    parse_as_string: List[str] = []
+) -> Union[DataBlock, Dict[DataBlock]]:
+    """Read data from a STAR file.
 
-    default behaviour in the case of only one data block being present in the STAR file is to
-    return only a dataframe, this can be changed by setting 'always_dict=True'
-    """
+    Basic data blocks are read as dictionaries. Loop blocks are read as pandas
+    dataframes. When multiple data blocks are present a dictionary of datablocks is
+    returned. When a single datablock is present only the block is returned by default.
+    To force returning a dectionary even when only one datablock is present set
+    `always_dict=True`.
 
-    parser = StarParser(filename, n_blocks_to_read=read_n_blocks)
+    Parameters
+    ----------
+    filename: PathLike
+        File from which to read data.
+    read_n_blocks: int | None
+        Limit reading the file to the first n data blocks.
+    always_dict: bool
+        Always return a dictionary, even when only a single data block is present.
+    parse_as_string: list[str]
+        A list of keys or column names which will not be coerced to numeric values.
+    """
+    parser = StarParser(filename, n_blocks_to_read=read_n_blocks, parse_as_string=parse_as_string)
     if len(parser.data_blocks) == 1 and always_dict is False:
         return list(parser.data_blocks.values())[0]
     else:
@@ -38,9 +55,24 @@ def write(
     na_rep: str = '<NA>',
     quote_character: str = '"',
     quote_all_strings: bool = False,
-    **kwargs,
+    **kwargs
 ):
-    """Write data blocks as STAR files."""
+    """Write data to disk in the STAR format.
+
+    Parameters
+    ----------
+    data: DataBlock | Dict[str, DataBlock] | List[DataBlock]
+        Data to be saved to file. DataBlocks are dictionaries or dataframes.
+        If a dictionary of datablocks are passed the keys will be the data block names.
+    filename: PathLike
+        Path where the file will be saved.
+    float_format: str
+        Float format string which will be passed to pandas.
+    sep: str
+        Separator between values, will be passed to pandas.
+    na_rep: str
+        Representation of null values, will be passed to pandas.
+    """
     StarWriter(
         data,
         filename=filename,

src/starfile/parser.py

@@ -8,27 +8,28 @@
 import numpy as np
 import pandas as pd
 from pathlib import Path
-from typing import TYPE_CHECKING, Union, Optional, Dict, Tuple
+from typing import TYPE_CHECKING, Union, Optional, Dict, Tuple, List
 
 from starfile.typing import DataBlock
 
 if TYPE_CHECKING:
     from os import PathLike
 
-def _apply_numeric(col: pd.Series) -> pd.Series:
-    try:
-        return pd.to_numeric(col)
-    except ValueError:
-        return col
 
 class StarParser:
     filename: Path
     n_lines_in_file: int
     n_blocks_to_read: int
     current_line_number: int
     data_blocks: Dict[DataBlock]
-
-    def __init__(self, filename: PathLike, n_blocks_to_read: Optional[int] = None):
+    parse_as_string: List[str]
+
+    def __init__(
+        self,
+        filename: PathLike,
+        n_blocks_to_read: Optional[int] = None,
+        parse_as_string: List[str] = [],
+    ):
         # set filename, with path checking
         filename = Path(filename)
         if not filename.exists():
@@ -39,6 +40,7 @@ def __init__(self, filename: PathLike, n_blocks_to_read: Optional[int] = None):
         self.data_blocks = {}
         self.n_lines_in_file = count_lines(self.filename)
         self.n_blocks_to_read = n_blocks_to_read
+        self.parse_as_string = parse_as_string
 
         # parse file
         self.current_line_number = 0
@@ -78,7 +80,15 @@ def _parse_simple_block(self) -> Dict[str, Union[str, int, float]]:
                 break
             elif self.current_line.startswith('_'):  # '_foo bar'
                 k, v = shlex.split(self.current_line)
-                block[k[1:]] = numericise(v)
+                column_name = k[1:]
+                parse_column_as_string = (
+                    self.parse_as_string is not None
+                    and any(column_name == col for col in self.parse_as_string)
+                )
+                if parse_column_as_string is True:
+                    block[column_name] = v
+                else:
+                    block[column_name] = numericise(v)
             self.current_line_number += 1
         return block
 
@@ -108,18 +118,27 @@ def _parse_loop_block(self) -> pd.DataFrame:
             n_cols = len(loop_column_names)
             df = pd.DataFrame(np.zeros(shape=(0, n_cols)))
         else:
+            column_name_to_index = {col: idx for idx, col in enumerate(loop_column_names)}
             df = pd.read_csv(
                 StringIO(loop_data.replace("'", '"')),
                 delimiter=r'\s+',
                 header=None,
                 comment='#',
-                keep_default_na=False
+                dtype={column_name_to_index[k]: str for k in self.parse_as_string if k in loop_column_names},
+                keep_default_na=False,
+                engine='c',
             )
+            df.columns = loop_column_names
+
+            # Numericise all columns in temporary copy
             df_numeric = df.apply(_apply_numeric)
-            # Replace columns that are all NaN with the original string columns
+
+            # Replace columns that are all NaN with the original columns
             df_numeric[df_numeric.columns[df_numeric.isna().all()]] = df[df_numeric.columns[df_numeric.isna().all()]]
-            df = df_numeric
-            df.columns = loop_column_names
+
+            # Replace columns that should be strings
+            for col in df.columns:
+                df[col] = df_numeric[col] if col not in self.parse_as_string else df[col]
         return df
 
 
@@ -150,3 +169,10 @@ def numericise(value: str) -> Union[str, int, float]:
             # If it's not a float either, leave it as a string
             value = value
     return value
+
+
+def _apply_numeric(col: pd.Series) -> pd.Series:
+    try:
+        return pd.to_numeric(col)
+    except ValueError:
+        return col

tests/test_parsing.py

@@ -243,34 +243,47 @@ def test_empty_loop_block():
     assert len(parser.data_blocks) == 1
 
 
-
-@pytest.mark.parametrize("quote_character, filename", [("'",basic_single_quote), 
-                                                 ('"',basic_double_quote), 
-                                                 ])
-def test_quote_basic(quote_character,filename):
+@pytest.mark.parametrize("quote_character, filename", [("'", basic_single_quote),
+                                                       ('"', basic_double_quote),
+                                                       ])
+def test_quote_basic(quote_character, filename):
     parser = StarParser(filename)
     assert len(parser.data_blocks) == 1
     assert parser.data_blocks['']['no_quote_string'] == "noquote"
     assert parser.data_blocks['']['quote_string'] == "quote string"
     assert parser.data_blocks['']['whitespace_string'] == " "
     assert parser.data_blocks['']['empty_string'] == ""
 
-@pytest.mark.parametrize("quote_character, filename", [("'",loop_single_quote), 
-                                                 ('"',loop_double_quote), 
-                                                 ])
-def test_quote_loop(quote_character,filename):
+
+@pytest.mark.parametrize("quote_character, filename", [("'", loop_single_quote),
+                                                       ('"', loop_double_quote),
+                                                       ])
+def test_quote_loop(quote_character, filename):
     import math
     parser = StarParser(filename)
     assert len(parser.data_blocks) == 1
-    assert parser.data_blocks[''].loc[0,'no_quote_string'] == "noquote"
-    assert parser.data_blocks[''].loc[0,'quote_string'] == "quote string"
-    assert parser.data_blocks[''].loc[0,'whitespace_string'] == " "
-    assert parser.data_blocks[''].loc[0,'empty_string'] == ""
+    assert parser.data_blocks[''].loc[0, 'no_quote_string'] == "noquote"
+    assert parser.data_blocks[''].loc[0, 'quote_string'] == "quote string"
+    assert parser.data_blocks[''].loc[0, 'whitespace_string'] == " "
+    assert parser.data_blocks[''].loc[0, 'empty_string'] == ""
 
     assert parser.data_blocks[''].dtypes['number_and_string'] == object
     assert parser.data_blocks[''].dtypes['number_and_empty'] == 'float64'
     assert parser.data_blocks[''].dtypes['number'] == 'float64'
     assert parser.data_blocks[''].dtypes['empty_string_and_normal_string'] == object
 
-    assert math.isnan(parser.data_blocks[''].loc[1,'number_and_empty'])
-    assert parser.data_blocks[''].loc[0,'empty_string_and_normal_string'] == ''
+    assert math.isnan(parser.data_blocks[''].loc[1, 'number_and_empty'])
+    assert parser.data_blocks[''].loc[0, 'empty_string_and_normal_string'] == ''
+
+
+def test_parse_as_string():
+    parser = StarParser(postprocess, parse_as_string=['rlnFinalResolution', 'rlnResolution'])
+
+    # check 'rlnFinalResolution' is parsed as string in general (basic) block
+    block = parser.data_blocks['general']
+    assert type(block['rlnFinalResolution']) == str
+
+    # check 'rlnResolution' is parsed as string in fsc (loop) block
+    df = parser.data_blocks['fsc']
+    assert df['rlnResolution'].dtype == 'object'
+