如果您有一个非常大的文件，并假设您希望输出为numpy数组，则使用np。Genfromtxt将冻结您的计算机。以我的经验来看，这样好多了:

def load_big_file(fname,maxrows):
'''only works for well-formed text file of space-separated doubles'''

rows = []  # unknown number of lines, so use list

with open(fname) as f:
    j=0        
    for line in f:
        if j==maxrows:
            break
        else:
            line = [float(s) for s in line.split()]
            rows.append(np.array(line, dtype = np.double))
            j+=1
return np.vstack(rows)  # convert list of vectors to array

Python 3:

with open("datafile") as myfile:
    head = [next(myfile) for x in range(N)]
print(head)

Python 2:

with open("datafile") as myfile:
    head = [next(myfile) for x in xrange(N)]
print head

下面是另一种方法(Python 2和3都是):

from itertools import islice

with open("datafile") as myfile:
    head = list(islice(myfile, N))
print(head)

fname = input("Enter file name: ")
num_lines = 0

with open(fname, 'r') as f: #lines count
    for line in f:
        num_lines += 1

num_lines_input = int (input("Enter line numbers: "))

if num_lines_input <= num_lines:
    f = open(fname, "r")
    for x in range(num_lines_input):
        a = f.readline()
        print(a)

else:
    f = open(fname, "r")
    for x in range(num_lines_input):
        a = f.readline()
        print(a)
        print("Don't have", num_lines_input, " lines print as much as you can")


print("Total lines in the text",num_lines)

我想通过读取整个文件来处理小于n行的文件

def head(filename: str, n: int):
    try:
        with open(filename) as f:
            head_lines = [next(f).rstrip() for x in range(n)]
    except StopIteration:
        with open(filename) as f:
            head_lines = f.read().splitlines()
    return head_lines

这要归功于约翰·拉·鲁伊和伊莲·伊利耶夫。使用异常句柄函数以获得最佳性能

修改1:感谢FrankM的反馈，处理文件存在和读取权限我们可以进一步增加

import errno
import os

def head(filename: str, n: int):
    if not os.path.isfile(filename):
        raise FileNotFoundError(errno.ENOENT, os.strerror(errno.ENOENT), filename)  
    if not os.access(filename, os.R_OK):
        raise PermissionError(errno.EACCES, os.strerror(errno.EACCES), filename)     
   
    try:
        with open(filename) as f:
            head_lines = [next(f).rstrip() for x in range(n)]
    except StopIteration:
        with open(filename) as f:
            head_lines = f.read().splitlines()
    return head_lines

您可以使用第二个版本，也可以使用第一个版本，稍后再处理文件异常。从性能的角度来看，检查是快速的，而且大部分是免费的

#!/usr/bin/python

import subprocess

p = subprocess.Popen(["tail", "-n 3", "passlist"], stdout=subprocess.PIPE)

output, err = p.communicate()

print  output

这个方法对我很有效

没有特定的方法来读取文件对象暴露的行数。

我想最简单的方法是:

lines =[]
with open(file_name) as f:
    lines.extend(f.readline() for i in xrange(N))