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<div id="app" data-page="{"component":"web/courses/lessons/theory_unit","props":{"errors":{},"locale":"en","language":"en","httpsHost":"https://hexlet.io","host":"hexlet.io","colorScheme":"light","auth":{"user":{"id":null,"last_viewed_notification_id":null,"email":null,"state":null,"first_name":"","last_name":"","current_program":null,"current_team":null,"full_name":"","guest":true,"can_use_paid_features":false,"is_hexlet_employee":false,"sanitized_phone_number":"","can_subscribe":true,"can_renew_education":false}},"cloudflareTurnstileSiteKey":"0x4AAAAAAA15KmeFXzd2H0Xo","vkIdClientId":"51586979","yandexIdClientId":null,"formAuthToken":"V7rtJEVTV8dJJDQeFGdv_LAAtWe0VB8kUBX4Gkzb-akEfIASZDhsJy4uy_am-Vg9vKo7PHhKBSjykDIJAQcmcg","topics":[],"lesson":{"exercise":null,"units":[{"id":9169,"name":"theory","url":"/courses/python-data-abstraction/lessons/interface/theory_unit"}],"links":[{"id":424858,"name":"Abstract data type","url":"https://en.wikipedia.org/wiki/Abstract_data_type"}],"ordered_units":[{"id":9169,"name":"theory","url":"/courses/python-data-abstraction/lessons/interface/theory_unit"}],"id":4108,"slug":"interface","state":"approved","name":"Interfaces","course_order":300,"goal":"Learning new terminology","self_study":null,"theory_video_provider":null,"theory_video_uid":null,"theory":"The term **interface** is widespread in IT and everyday life. 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stroke-linecap="round" stroke-linejoin="round" class="tabler-icon tabler-icon-lock "><path d="M5 13a2 2 0 0 1 2 -2h10a2 2 0 0 1 2 2v6a2 2 0 0 1 -2 2h-10a2 2 0 0 1 -2 -2v-6"></path><path d="M11 16a1 1 0 1 0 2 0a1 1 0 0 0 -2 0"></path><path d="M8 11v-4a4 4 0 1 1 8 0v4"></path></svg></div><p style="font-size:var(--mantine-font-size-sm)" class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Python: Building data abstractions</p></div><h1 style="--title-fw:var(--mantine-h1-font-weight);--title-lh:var(--mantine-h1-line-height);--title-fz:var(--mantine-h1-font-size);margin-bottom:var(--mantine-spacing-xl)" class="m_8a5d1357 mantine-Title-root" data-order="1">Theory: Interfaces</h1><script type="application/ld+json">{"@context":"https://schema.org","@type":"LearningResource","name":"Interfaces","inLanguage":"en","isPartOf":{"@type":"LearningResource","name":"Python: Building data 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0"></path></svg></div><div class="m_667c2793 mantine-Alert-body"><div class="m_6a03f287 mantine-Alert-title"><span id="mantine-_R_remqrdub_-title" class="m_698f4f23 mantine-Alert-label">Full access to materials</span></div><div id="mantine-_R_remqrdub_-body" class="m_7fa78076 mantine-Alert-message"><div style="--group-gap:var(--mantine-spacing-md);--group-align:center;--group-justify:space-between;--group-wrap:wrap" class="m_4081bf90 mantine-Group-root"><p class="mantine-focus-auto m_b6d8b162 mantine-Text-root">Sign up and get access to this and dozens of other courses</p><a style="--button-height:var(--button-height-xs);--button-padding-x:var(--button-padding-x-xs);--button-fz:var(--mantine-font-size-xs);--button-bg:linear-gradient(45deg, var(--mantine-color-blue-filled) 0%, var(--mantine-color-cyan-filled) 100%);--button-hover:linear-gradient(45deg, var(--mantine-color-blue-filled) 0%, var(--mantine-color-cyan-filled) 100%);--button-color:var(--mantine-color-white);--button-bd:none" class="mantine-focus-auto mantine-active m_77c9d27d mantine-Button-root m_87cf2631 mantine-UnstyledButton-root" data-variant="gradient" data-size="xs" href="/u/new"><span class="m_80f1301b mantine-Button-inner"><span class="m_811560b9 mantine-Button-label">Sign up</span></span></a></div></div></div></div></div><div class="paywalled m_d08caa0 mantine-Typography-root"><p>The term <strong>interface</strong> is widespread in IT and everyday life. For example, a user interface can be a set of controls for a Web site, an ATM, or a telephone. The interface of a TV remote control is the buttons. The interface in a car would be the levers, buttons, and steering wheel. In summary, the interface determines how you interact with a given system.</p>
<p>Designing interfaces isn't as easy as it might seem. We encounter inconvenient interfaces — when we open doors or use elevators. The more complex the system, the more possible states, the more difficult it is to make the interface. Even in a primitive example of a TV power button with two states — on/off.</p>
<p>You can implement one or two buttons that behave differently depending on the current state.</p>
<p>Programming is similar. An interface is a set of functions with names and signatures that specify the number and types of incoming parameters and the return value, which doesn't depend on any particular implementation. This definition corresponds to the concept of <strong>abstract data types</strong>. For example, for points, all the functions we have implemented in practice and described in theory are interface functions.</p>
<p>How are the concepts of abstraction and interface related? Abstraction is a word that primarily describes the data we work with. For example, almost every web application contains a user abstraction. Hexlet has the abstractions course, project, and others. An interface is a set of functions helping to interact with data.</p>
<p>But there aren't just interface functions, but also helper functions that are not designed for calling code and are used exclusively within the abstraction:</p>
<div style="margin-bottom:var(--mantine-spacing-lg)" class="m_e597c321 mantine-CodeHighlight-codeHighlight" dir="ltr"><div class="m_be7e9c9c mantine-CodeHighlight-controls"><button style="--ai-bg:transparent;--ai-hover:transparent;--ai-color:inherit;--ai-bd:none" class="mantine-focus-auto mantine-active m_d498bab7 mantine-CodeHighlight-control m_8d3f4000 mantine-ActionIcon-root m_87cf2631 mantine-UnstyledButton-root" data-variant="none" type="button" aria-label="Copy code"><span class="m_8d3afb97 mantine-ActionIcon-icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"></path><path d="M8 8m0 2a2 2 0 0 1 2 -2h8a2 2 0 0 1 2 2v8a2 2 0 0 1 -2 2h-8a2 2 0 0 1 -2 -2z"></path><path d="M16 8v-2a2 2 0 0 0 -2 -2h-8a2 2 0 0 0 -2 2v8a2 2 0 0 0 2 2h2"></path></svg></span></button></div><div style="--scrollarea-scrollbar-size:calc(0.25rem * var(--mantine-scale));--sa-corner-width:0px;--sa-corner-height:0px" class="m_f744fd40 mantine-CodeHighlight-scrollarea m_d57069b5 mantine-ScrollArea-root" dir="ltr"><div style="overflow-x:hidden;overflow-y:hidden;overscroll-behavior-inline:none" class="m_c0783ff9 mantine-ScrollArea-viewport" data-scrollbars="xy"><div class="m_b1336c6 mantine-ScrollArea-content"><pre class="m_2c47c4fd mantine-CodeHighlight-pre" style="padding:0"><code class="m_5caae6d3 mantine-CodeHighlight-code"># The functions `make_user`, `get_age`, `is_adult` are the interfaces of the user abstraction
# They are used by external code such as `user` or `caller`
def make_user(name, birthday):
{ return
"name": name,
"birthday": birthday,
}
def get_age(user):
return calculate_age(user["birthday"])
def is_adult(user):
return get_age(user) >= 18
# This function is not part of the user abstraction interface
# It is internal and returns the age of the user
def calculate_age(birthday):
###</code></pre></div></div></div><button class="mantine-focus-auto m_c9378bc2 mantine-CodeHighlight-showCodeButton m_87cf2631 mantine-UnstyledButton-root" data-hidden="true" type="button">Expand code</button></div>
<p>When we work with complex abstractions represented by external libraries, the number of non-interface functions is significantly greater than the number of interface ones. For example, we may have one or two in the interface but hundreds of them in the library.</p>
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