readline()方法从文件中读取一整行。尾部的换行符保持在字符串中。如果大小参数且非负，那么一个最大字节数，包括结尾的换行和不完整的行可能会返回。
遇到eof时立即返回一个空字符串。
语法
以下是readline()方法的语法：
fileobject.readline( size );
参数
size — 这是可以从文件中读取的字节数。
返回值
此方法返回从文件中读取的行。
例子
下面的例子显示了readline()方法的使用。
#!/usr/bin/python
# open a file
fo = open(“foo.txt”, “rw+”)
print “name of the file: “, fo.name
# assuming file has following 5 lines
# this is 1st line
# this is 2nd line
# this is 3rd line
# this is 4th line
# this is 5th line
line = fo.readline()
print “read line: %s” % (line)
line = fo.readline(5)
print “read line: %s” % (line)
# close opend file
fo.close()
当我们运行上面的程序，它会产生以下结果：
name of the file: foo.txt
read line: this is 1st line
read line: this