{"id":5118,"date":"2024-09-18T10:36:42","date_gmt":"2024-09-18T14:36:42","guid":{"rendered":"https:\/\/ikriv.com\/blog\/?p=5118"},"modified":"2024-09-18T10:36:42","modified_gmt":"2024-09-18T14:36:42","slug":"python-generator-expression-can-be-iterated-only-once","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=5118","title":{"rendered":"Python generator expression can be iterated only once"},"content":{"rendered":"<p><b>TL;DR<\/b> Python generator expressions can be iterated over only once.<\/p>\n<pre class=\"brush: python; title: ; notranslate\" title=\"\">\r\nsquares = (x*x for x in range(10))  # generator expression\r\nprint(list(squares))                # &#x5B;0, 1, 4, 9, 16, 25, 36, 49, 64, 81]\r\nprint(list(squares))                # &#x5B;]\r\n<\/pre>\n<p>I mentally mapped Python generator expressions to C# <code>IEnumerable<\/code>s, which can be iterated over multiple times. In reality, Python generator expressions behave differently: they can be iterated only once, and then just silently stop yielding values, as demonstrated in the example above.<\/p>\n<p><a href=\"https:\/\/docs.python.org\/3\/reference\/expressions.html#generator-expressions\">Python documentation<\/a> simply says that generator expression &#8220;yields a new generator object&#8221;, but it does not specifically mention that it can be iterated only once.<\/p>\n<p><a href=\"https:\/\/stackoverflow.com\/questions\/40935272\/python-iterate-an-iterator-twice\">This Stackoverflow article<\/a> specifically contrasts Python and C# generators.<\/p>\n<p>It is possible to create multiple iterators over the same generator expression by using <a href=\"https:\/\/docs.python.org\/3\/library\/itertools.html#itertools.tee\">itertools.tee()<\/a>, but it seems to buffer the output in memory, so if the generator returns different values on different invocations, it won&#8217;t work properly. Also, you need to know in advance, how many iterators you want, there is no way to create new iterators on the fly.<\/p>\n<pre class=\"brush: python; title: ; notranslate\" title=\"\">\r\nimport itertools\r\nsquares = (x*x for x in range(10))\r\ntees = itertools.tee(squares,3)\r\nfor iter in tees:\r\n    print(list(iter))\r\n<\/pre>\n<p>This prints<\/p>\n<pre>\r\n[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]\r\n[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]\r\n[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]\r\n<\/pre>\n<p>In conclusion, one must be careful when using generator expressions in Python, since they can be iterated only once. In retrospect, I had some mysterious bugs related to this, which disappeared when I replaced generator expressions with lists.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>TL;DR Python generator expressions can be iterated over only once. squares = (x*x for x in range(10)) # generator expression print(list(squares)) # &#x5B;0, 1, 4, 9, 16, 25, 36, 49, <a href=\"https:\/\/ikriv.com\/blog\/?p=5118\" class=\"more-link\">[&hellip;]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"Layout":"","footnotes":""},"categories":[4,26],"tags":[],"class_list":["entry","author-ikriv","post-5118","post","type-post","status-publish","format-standard","category-hack","category-python"],"_links":{"self":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5118","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5118"}],"version-history":[{"count":1,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5118\/revisions"}],"predecessor-version":[{"id":5119,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5118\/revisions\/5119"}],"wp:attachment":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5118"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5118"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5118"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}