我想用pandas解析一些数据时只选择2列。
pd.read_table的帮助提到usecols选项似乎正是我想要的：
usecols : array-like, default None
Return a subset of the columns. All elements in this array must either
be positional (i.e. integer indices into the document columns) or strings
that correspond to column names provided either by the user in `names` or
inferred from the document header row(s). For example, a valid `usecols`
parameter would be [0, 1, 2] or ['foo', 'bar', 'baz']. Using this parameter
results in much faster parsing time and lower memory usage.
我的数据一旦读取，就会显示编号为0到6的列：
In [338]: pd.read_table("../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/intersect_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt", index_
...: col=3, header=None)[:3]
Out[338]:
0 1 2 4 5 6
3
WBGene00022277 I 4118 10230 - . 83
WBGene00022276 I 10412 16842 + . 230
WBGene00022278 I 17482 26781 - . 303
但是当我尝试仅保留索引（第3列）和最后一个（第6列）时，我收到以下错误：
In [339]: pd.read_table("../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/intersect_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt", index_
...: col=3, header=None, usecols=(3, 6))[:3]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-339-279bef505f16> in <module>()
----> 1 pd.read_table("../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/intersect_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt", index_col=3, header=None, usecols=(3, 6))[:3]
/home/bli/.local/lib/python3.6/site-packages/pandas/io/parsers.py in parser_f(filepath_or_buffer, sep, delimiter, header, names, index_col, usecols, squeeze, prefix, mangle_dupe_cols, dtype, engine, converters, true_values, false_values, skipinitialspace, skiprows, nrows, na_values, keep_default_na, na_filter, verbose, skip_blank_lines, parse_dates, infer_datetime_format, keep_date_col, date_parser, dayfirst, iterator, chunksize, compression, thousands, decimal, lineterminator, quotechar, quoting, escapechar, comment, encoding, dialect, tupleize_cols, error_bad_lines, warn_bad_lines, skipfooter, skip_footer, doublequote, delim_whitespace, as_recarray, compact_ints, use_unsigned, low_memory, buffer_lines, memory_map, float_precision)
644 delim_whitespace=delim_whitespace,
645 as_recarray=as_recarray,
--> 646 warn_bad_lines=warn_bad_lines,
647 error_bad_lines=error_bad_lines,
648 low_memory=low_memory,
/home/bli/.local/lib/python3.6/site-packages/pandas/io/parsers.py in _read(filepath_or_buffer, kwds)
387 kwds['encoding'] = encoding
388
--> 389 compression = kwds.get('compression')
390 compression = _infer_compression(filepath_or_buffer, compression)
391 filepath_or_buffer, _, compression = get_filepath_or_buffer(
/home/bli/.local/lib/python3.6/site-packages/pandas/io/parsers.py in __init__(self, f, engine, **kwds)
728
729 if dialect_val != provided:
--> 730 conflict_msgs.append((
731 "Conflicting values for '{param}': '{val}' was "
732 "provided, but the dialect specifies '{diaval}'. "
/home/bli/.local/lib/python3.6/site-packages/pandas/io/parsers.py in _make_engine(self, engine)
921 for arg in _deprecated_args:
922 parser_default = _c_parser_defaults[arg]
--> 923 msg = ("The '{arg}' argument has been deprecated "
924 "and will be removed in a future version."
925 .format(arg=arg))
/home/bli/.local/lib/python3.6/site-packages/pandas/io/parsers.py in __init__(self, src, **kwds)
1445 cast_type = dtypes
1446
-> 1447 if self.na_filter:
1448 col_na_values, col_na_fvalues = _get_na_values(
1449 c, na_values, na_fvalues)
/home/bli/.local/lib/python3.6/site-packages/pandas/io/parsers.py in _clean_index_names(columns, index_col)
2812 msg = ('Expected %d fields in line %d, saw %d' %
2813 (col_len, row_num + 1, actual_len))
-> 2814 if len(self.delimiter) > 1 and self.quoting != csv.QUOTE_NONE:
2815 # see gh-13374
2816 reason = ('Error could possibly be due to quotes being '
IndexError: list index out of range
我在另一种情况下成功使用了usecols选项，但保留了原始文件中的一些标题。
造成问题的原因是什么？
header=None显然不是问题我可以解析格式不同的文件，而不保留标题，usecols选项有效：
In [361]: pd.read_table("../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/feature_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt", skiprows
...: =2, index_col=0, header=None, usecols=[0, 6])[:3]
Out[361]:
6
0
WBGene00022277 72
WBGene00022276 222
WBGene00022278 302
答案 0 :(得分：1)
我看起来与index_col
尝试读取文件后设置索引：
path = "../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/intersect_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt"
df = pd.read_table(path, header=None, usecols=(3, 6)).set_index(3)[:3]
显然在减少列后使用index_col。您正在选择两列，然后尝试选择第三列作为索引。
path = "../RNA_Seq_analyses/mapping_worm_number_tests/hisat2/mapped_C_elegans/intersect_count/W100_1_on_C_elegans/protein_coding_fwd_counts.txt"
df = pd.read_table(path, header=None, usecols=(3, 6), index_col=0)[:3]