{"id":6491,"date":"2018-10-22T02:04:21","date_gmt":"2018-10-22T02:04:21","guid":{"rendered":"https:\/\/www.jquery-az.com\/?p=6491"},"modified":"2018-10-22T02:04:21","modified_gmt":"2018-10-22T02:04:21","slug":"python-readline","status":"publish","type":"post","link":"https:\/\/www.jquery-az.com\/python-readline\/","title":{"rendered":"What is Python file readline() method?"},"content":{"rendered":"<p><strong>A few main points about the readline() method:<\/strong><\/p>\n<ul>\n<li>The readline() method reads single line from the specified file.<\/li>\n<li>If used in text mode then readline() returns string and returns byte object in binary mode.<\/li>\n<li>A \u201c<strong>\\n\u201d<\/strong>, the trailing newline character is left at the end of the string.<\/li>\n<li>You may provide <strong>size<\/strong> argument in the readline() method, the optional numeric argument.<\/li>\n<li>If you do not provide the size argument or a <strong>negative value<\/strong> is given then the entire content of the file is read and returned.<\/li>\n<li>If the size argument is given then it is the maximum byte count. In that case, the newline character is also included and readline() method may return an incomplete line.<\/li>\n<\/ul>\n<h3><strong>How to use the readline() method?<\/strong><\/h3>\n<p>The general syntax for using the readline() method is:<\/p>\n<blockquote><p>FO.readline( size );<\/p><\/blockquote>\n<p>Where FO is the file object.<\/p>\n<h3><strong>An example of readline() with text file without size<\/strong><\/h3>\n<p>In the first example, I am using readline() method to return the content of a text file without specifying the size argument. So, the complete content should be returned and I will execute readline() only once to display the content.<\/p>\n<p>For the clarity, the text file contains following demo content for all our examples below:<\/p>\n<blockquote><p>Python<\/p>\n<p>Open<\/p>\n<p>Read<\/p>\n<p>Write<\/p>\n<p>Close<\/p><\/blockquote>\n<p>The text file name is <em>python-demo.txt<\/em> and this is stored at the same location where our code file is placed.<\/p>\n<p><strong>The code:<\/strong><\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\"># readline() example\r\n\r\n\r\n\r\nobjFile = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nprint (objFile.readline());\r\n\r\n\r\n\r\nobjFile.close()<\/pre>\n<p><strong>The output:<\/strong><\/p>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6492\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.1-Python-readline-simple.png\" alt=\"Python readline simple\" width=\"269\" height=\"89\" \/><\/p>\n<p>You can see only the first line is displayed along with the \u201c\\n\u201d character i.e. a line.<\/p>\n<h3><strong>The example of specifying the size argument<\/strong><\/h3>\n<p>In the following example, I provide the value <strong>2<\/strong> for the size argument in the readline() method. See what it returns:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\"># readline() example with size\r\n\r\n\r\n\r\nobjFile_size = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nprint (objFile_size.readline(2));\r\n\r\n\r\n\r\nobjFile_size.close()<\/pre>\n<p><strong>The output:<\/strong><\/p>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6493\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.2-Python-readline-size.png\" alt=\"Python readline size\" width=\"252\" height=\"67\" \/><\/p>\n<p>The readline() method only returned first two characters while the newline is also not included. If we specify the size 7 in the above example then it should return a newline as shown below:<\/p>\n<blockquote><p>print (objFile_size.readline(7));<\/p><\/blockquote>\n<p><strong>The output:<\/strong><\/p>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6494\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.3-Python-readline.png\" alt=\"Python readline\" width=\"230\" height=\"86\" \/><\/p>\n<h3><strong>Reading next lines in text file example<\/strong><\/h3>\n<p>The example below displays the first four lines from the demo text file including \u201c\\n\u201d with each line. For that, the readline() method is used four times that will read the file line by line as follows:<\/p>\n<p><strong>The code:<\/strong><\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\"># readline() example with size\r\n\r\n\r\n\r\nobjFile_multiple = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nprint (objFile_multiple.readline());\r\n\r\nprint (objFile_multiple.readline());\r\n\r\nprint (objFile_multiple.readline());\r\n\r\nprint (objFile_multiple.readline());\r\n\r\n\r\n\r\nobjFile_multiple.close()<\/pre>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6495\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.4-Python-readline-multi.png\" alt=\"Python readline multiple\" width=\"219\" height=\"181\" \/><\/p>\n<h3><strong>Using a for loop to display content<\/strong><\/h3>\n<p>The memory efficient and fast way of reading the complete content from the file line by line is using a loop and iterate through multiple lines.<\/p>\n<p>The example below uses a <a href=\"\/learn-using-python-for-loop-7-examples\/\">for loop<\/a> with the file object (same file as in the above examples) and displays the complete content of the file. Have a look:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\"># readline() example with loop\r\n\r\n\r\n\r\nobjFile_loop = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nfor line in objFile_loop:\r\n\r\n\u00a0\u00a0\u00a0 print(line)\r\n\r\n\r\n\r\n\r\n\r\nobjFile_loop.close()<\/pre>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6496\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.5-Python-readline-loop.png\" alt=\"Python readline loop\" width=\"233\" height=\"176\" \/><\/p>\n<p>Just like the above examples, you may notice a line break is added.<\/p>\n<p>You may also omit the newline character from displaying as shown in the example below (with for loop):<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\">objFile_loop_omit = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nfor line in objFile_loop_omit:\r\n\r\n\u00a0\u00a0\u00a0 print(line, end='')\r\n\r\n\r\n\r\nobjFile_loop_omit.close()<\/pre>\n<p><strong>The result:<\/strong><\/p>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6497\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.6-Python-readline-newlin.png\" alt=\"Python readline newline\" width=\"201\" height=\"111\" \/><\/p>\n<p>You can see no gap between the lines in the above graphic.<\/p>\n<h3><strong>What if size is given as negative?<\/strong><\/h3>\n<p>As mentioned earlier, you may provide the value for size argument as negative or do not provide at all then readline() method returns complete file content.<\/p>\n<p>You have seen an example in the above section with the positive value of 2 and it returned only two characters, now have a look at what happens when I used <strong>-2<\/strong> value:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\"># readline() example with a negative number\r\n\r\n\r\n\r\nobjFileSizeNeg = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nprint (objFileSizeNeg.readline(-2));\r\n\r\n\r\n\r\nobjFileSizeNeg.close()<\/pre>\n<p><strong>The result:<\/strong><\/p>\n<p><img decoding=\"async\" loading=\"lazy\" class=\"aligncenter size-full wp-image-6498\" src=\"https:\/\/www.jquery-az.com\/wp-content\/uploads\/2018\/10\/62.7-readline-negative.png\" alt=\"readline negative\" width=\"201\" height=\"66\" \/><\/p>\n<p>The first complete line is displayed including a newline character. That means, the complete content is returned by readline() method, although, we provided -2 value.<\/p>\n<h3><strong>Using list to display file content (not line by line)<\/strong><\/h3>\n<p>You may also use list(file_object) for reading the file\u2019s content completely. The example below shows using the <a href=\"\/python-list-12-examples-to-learn-with-built-in-methods\/\">list<\/a> and there you may also see \u201c\\n\u201d is added with each element in the list:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"python\">objFileSizeNeg = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nprint (list(objFileSizeNeg));\r\n\r\nobjFileSizeNeg.close()<\/pre>\n<p>The output of the above code:<\/p>\n<blockquote><p>[&#8216;Python\\n&#8217;, &#8216;Open\\n&#8217;, &#8216;Read\\n&#8217;, &#8216;Write\\n&#8217;, &#8216;Close&#8217;]<\/p><\/blockquote>\n<p>The same is displayed as you use readlines() method for displaying the complete content of the file:<\/p>\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"kotlin\">objFile = open(\"python-demo.txt\", \"r\")\r\n\r\n\r\n\r\nprint (objFile.readlines())\r\n\r\n\r\n\r\nobjFile.close()<\/pre>\n<p><strong>The result:<\/strong><\/p>\n<blockquote><p>[&#8216;Python\\n&#8217;, &#8216;Open\\n&#8217;, &#8216;Read\\n&#8217;, &#8216;Write\\n&#8217;, &#8216;Close&#8217;]<\/p><\/blockquote>\n","protected":false},"excerpt":{"rendered":"<p>Python readline() method reads single line from the specified file. 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