如何查找制表符分隔文件中的列数

Question

我有一个制表符分隔的文件，其中包含 10 亿行（想象一下 200 多列而不是 3 列）：

abc -0.123  0.6524  0.325
foo -0.9808 0.874   -0.2341 
bar 0.23123 -0.123124   -0.1232

如果列数未知，如何查找制表符分隔文件中的列数？

我试过了：

import io
with io.open('bigfile', 'r') as fin:
    num_columns = len(fin.readline().split('\t'))

并且（来自@EdChum，Read a tab separated file with first column as key and the rest as values）：

import pandas as pd
num_columns = pd.read_csv('bigfile', sep='\s+', nrows=1).shape[1]  

我还能如何获得列数？哪种方式最有效？（想象一下，我突然收到一个列数未知的文件，比如超过 100 万列）

Answer 1

有一个str.count()方法：

h = file.open('path', 'r')
columns = h.readline().count('\t') + 1
h.close()

Answer 2

一个有 100000 列的文件的一些计时，count 似乎最快，但相差 1：

In [14]: %%timeit                    
with open("test.csv" ) as f:
    r = csv.reader(f, delimiter="\t")
    len(next(r))
   ....: 
10 loops, best of 3: 88.7 ms per loop

In [15]: %%timeit                    
with open("test.csv" ) as f:
    next(f).count("\t")
   ....: 
100 loops, best of 3: 11.9 ms per loop
with io.open('test.csv', 'r') as fin:
    num_columns = len(next(fin).split('\t'))
    ....: 
 10 loops, best of 3: 133 ms per loop

实际上使用 str.translate 似乎是最快的，尽管您需要再次添加 1：

In [5]: %%timeit
with open("test.csv" ) as f:
    n = next(f)
    (len(n) - len(n.translate(None, "\t")))
   ...: 
100 loops, best of 3: 9.9 ms per loop

pandas 解决方案给我一个错误：

in pandas.parser.TextReader._read_low_memory (pandas/parser.c:7977)()

StopIteration: 

使用 readline 会增加更多开销：

In [19]: %%timeit
with open("test.csv" ) as f:
    f.readline().count("\t")
   ....: 
10 loops, best of 3: 28.9 ms per loop
In [30]: %%timeit
with io.open('test.csv', 'r') as fin:
    num_columns = len(fin.readline().split('\t'))
   ....: 
10 loops, best of 3: 136 ms per loop

使用 python 3.4 的不同结果：

In [7]: %%timeit
with io.open('test.csv', 'r') as fin:
    num_columns = len(next(fin).split('\t'))
   ...: 
10 loops, best of 3: 102 ms per loop

In [8]: %%timeit
with open("test.csv" ) as f:
    f.readline().count("\t")
   ...: 

100 loops, best of 3: 12.7 ms per loop   
In [9]:     
In [9]: %%timeit
with open("test.csv" ) as f:
    next(f).count("\t")
   ...: 
100 loops, best of 3: 11.5 ms per loop    
In [10]: %%timeit
with io.open('test.csv', 'r') as fin:
    num_columns = len(next(fin).split('\t'))
   ....: 
10 loops, best of 3: 89.9 ms per loop    
In [11]: %%timeit
with io.open('test.csv', 'r') as fin:
    num_columns = len(fin.readline().split('\t'))
   ....: 
10 loops, best of 3: 92.4 ms per loop   
In [13]: %%timeit     
with open("test.csv" ) as f:
    r = csv.reader(f, delimiter="\t")
    len(next(r))
   ....: 
10 loops, best of 3: 176 ms per loop