Python 按行读取文本文件 缓存 和 非缓存实现

需求

最近项目中有个读取文件的需求，数据量还挺大，10万行的数量级。

java 使用缓存读取文件是，会相应的创建一个内部缓冲区数组在java虚拟机内存中，因此每次处理的就是这一整块内存。

简单的想：就是如果不用缓存，每次都要硬盘–虚拟机缓存–读取；有了缓存，提前读了一段放在虚拟机缓存里，可以避免频繁将硬盘上的数据读到缓存里。

因为对内存的操作肯定是比硬盘的操作要快的。

对了，java还有映射内存，可以解决大文件读写的问题。

思路

大文件读写不能一次全部读入内存，这样会导致耗尽内存。（但是在内存允许的情况下，全部读入内存是不是速度更快？？） 
对于大文件可以一行一行读取，因为我们处理完这行，就可以把它抛弃。

我们也可以一段一段读取大文件，实现一种缓存处理。每次读取一段文件，将这段文件放在缓存里，然后对这段处理。这会比一行一行快些。

方法1：一行一行读取

我们可以打开一个文件，然后用for循环读取每行，比如：

<code class="hljs python has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">method1</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(newName)</span>:</span>    s1 = time.clock()    oldLine = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'0'</span>    count = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> line <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> open(newName):        newLine =  line        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newLine != oldLine):            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#判断是不是空行</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> newLine.strip():                nu = newLine.split()[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]                oldLine = newLine                count += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"deal %s lines"</span> %(count)    e1 = time.clock()    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cost time "</span> + str(e1-s1)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

我们测试一下

<code class="hljs tex has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">fileName = 'E:<span class="hljs-command" style="box-sizing: border-box; color: rgb(0, 0, 136);">\\</span>pythonProject<span class="hljs-command" style="box-sizing: border-box; color: rgb(0, 0, 136);">\\</span>ruisi<span class="hljs-command" style="box-sizing: border-box; color: rgb(0, 0, 136);">\\</span>correct_re.txt'method1(fileName)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

输出

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">deal <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">218376</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">lines</span>cost <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">time</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.288900734402</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

方法1.1 一行一行读取的变形

<code class="hljs python has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">method11</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(newName)</span>:</span>    s1 = time.clock()    oldLine = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'0'</span>    count = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>    file = open(newName)    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:        line = file.readline()        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">not</span> line:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> line.strip():                newLine =  line                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newLine != oldLine):                    nu = newLine.split()[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]                    oldLine = newLine                    count += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"deal %s lines"</span> %(count)    e1 = time.clock()    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cost time "</span> + str(e1-s1)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">deal <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">218376</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">lines</span>cost <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">time</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.371977884619</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

耗时和方法1差不多，比方法1稍微多些。

方法2：一行一行，使用fileinput模块

<code class="hljs python has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">method2</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(newName)</span>:</span>    s1 = time.clock()    oldLine = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'0'</span>    count = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> line <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> fileinput.input(newName):        newLine =  line        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> newLine.strip():            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newLine != oldLine):                nu = newLine.split()[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]                oldLine = newLine                count += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"deal %s lines"</span> %(count)    e1 = time.clock()    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cost time "</span> + str(e1-s1)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">deal <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">218376</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">lines</span>cost <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">time</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.514534051673</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

这儿的耗时差不多是方法1的两倍。

借助缓存，每次读取1000行

<code class="hljs python has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">method3</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(newName)</span>:</span>    s1 = time.clock()    file = open(newName)    oldLine = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'0'</span>    count = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:        lines = file.readlines(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>*<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>)        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#print len(lines)</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">not</span> lines:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> line <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> lines:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> line.strip():                newLine =  line                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newLine != oldLine):                    nu = newLine.split()[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]                    oldLine = newLine                    count += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"deal %s lines"</span> %(count)    e1 = time.clock()</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

Note 
readlinessizehint() 参数是限定字节大小，不是行数。 
注意默认有个内部缓冲区大小是8KB，如果设定值小于 8*1024。那么都是按照8KB来的。print len(lines)输出大概都为290。 
只有当设定值大于8KB，上面的print len(lines)才会发生变化。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">deal <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">218376</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">lines</span>cost <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">time</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.296652349397</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

这儿的性能还没方法1，表现好。可以调整每次读取的行数，比如500,1000等等，可以达到不同的耗时。

方法4 一次性全部读到内存里

<code class="hljs python has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">method4</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(newName)</span>:</span>    s1 = time.clock()    file = open(newName)    oldLine = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'0'</span>    count = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> line <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> file.readlines():        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> line.strip():            newLine =  line            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newLine != oldLine):                nu = newLine.split()[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]                oldLine = newLine                count += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"deal %s lines"</span> %(count)    e1 = time.clock()    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cost time "</span> + str(e1-s1)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

输出

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">deal <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">218376</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">lines</span>cost <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">time</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.30108883108</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

结论

推荐使用

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-operator" style="box-sizing: border-box;">with</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">open</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'foo.txt'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'r'</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> f:    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">line</span> <span class="hljs-operator" style="box-sizing: border-box;">in</span> f:        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># do_something(line)</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

对于大文件可以使用索引，这个索引记录下每行开头的位置，之后就可以用file.seek()定位了。如果文件内容修改了，还需要重新建立索引。这个索引可以有很多种方法建立，但是都需要将文件遍历一次。

参考资料

python的readlines返回行数问题

Python按行读文件