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  Names
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<h1 id="Names">Names<a class="anchor-link" href="#Names"> </a></h1><p>Names are given to values in Python using an <em>assignment</em> statement. In an assignment, a name is followed by <code>=</code>, which is followed by any expression. The value of the expression to the right of <code>=</code> is <em>assigned</em> to the name. Once a name has a value assigned to it, the value will be substituted for that name in future expressions.</p>

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<div class=" highlight hl-ipython3"><pre><span></span><span class="n">a</span> <span class="o">=</span> <span class="mi">10</span>
<span class="n">b</span> <span class="o">=</span> <span class="mi">20</span>
<span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
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<pre>30</pre>
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<p>A previously assigned name can be used in the expression to the right of <code>=</code>.</p>

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<div class=" highlight hl-ipython3"><pre><span></span><span class="n">quarter</span> <span class="o">=</span> <span class="mi">1</span><span class="o">/</span><span class="mi">4</span>
<span class="n">half</span> <span class="o">=</span> <span class="mi">2</span> <span class="o">*</span> <span class="n">quarter</span>
<span class="n">half</span>
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<pre>0.5</pre>
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<p>However, only the current value of an expression is assigned to a name. If that value changes later, names that were defined in terms of that value will not change automatically.</p>

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<div class=" highlight hl-ipython3"><pre><span></span><span class="n">quarter</span> <span class="o">=</span> <span class="mi">4</span>
<span class="n">half</span>
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<pre>0.5</pre>
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<p>Names must start with a letter, but can contain both letters and numbers. A name cannot contain a space; instead, it is common to use an underscore character <code>_</code> to replace each space. Names are only as useful as you make them; it's up to the programmer to choose names that are easy to interpret. Typically, more meaningful names can be invented than <code>a</code> and <code>b</code>. For example, to describe the sales tax on a $5 purchase in Berkeley, CA, the following names clarify the meaning of the various quantities involved.</p>

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<div class=" highlight hl-ipython3"><pre><span></span><span class="n">purchase_price</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">state_tax_rate</span> <span class="o">=</span> <span class="mf">0.075</span>
<span class="n">county_tax_rate</span> <span class="o">=</span> <span class="mf">0.02</span>
<span class="n">city_tax_rate</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">sales_tax_rate</span> <span class="o">=</span> <span class="n">state_tax_rate</span> <span class="o">+</span> <span class="n">county_tax_rate</span> <span class="o">+</span> <span class="n">city_tax_rate</span>
<span class="n">sales_tax</span> <span class="o">=</span> <span class="n">purchase_price</span> <span class="o">*</span> <span class="n">sales_tax_rate</span>
<span class="n">sales_tax</span>
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<pre>0.475</pre>
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