-
Notifications
You must be signed in to change notification settings - Fork 252
/
Copy pathmutable_default_value_as_argument.html
254 lines (171 loc) · 15.1 KB
/
mutable_default_value_as_argument.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
<!DOCTYPE html>
<!--[if IE 8]><html class="no-js lt-ie9" lang="en" > <![endif]-->
<!--[if gt IE 8]><!--> <html class="no-js" lang="en" > <!--<![endif]-->
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Using a mutable default value as an argument — Python Anti-Patterns documentation</title>
<link rel="stylesheet" href="../_static/css/theme.css" type="text/css" />
<link rel="stylesheet" href="../_static/css/ribbon.css" type="text/css" />
<link rel="stylesheet" href="../_static/css/font-awesome-4.1.0/css/font-awesome.min.css" type="text/css" />
<link rel="stylesheet" href="../_static/css/menu.css" type="text/css" />
<link rel="index" title="Index"
href="../genindex.html"/>
<link rel="search" title="Search" href="../search.html"/>
<link rel="top" title="Python Anti-Patterns documentation" href="../index.html"/>
<link rel="up" title="Correctness" href="index.html"/>
<link rel="next" title="No exception type(s) specified" href="no_exception_type_specified.html"/>
<link rel="prev" title="Missing argument to super()" href="missing_argument_to_super.html"/>
</head>
<body class="wy-body-for-nav" role="document">
<div class="wy-grid-for-nav">
<nav data-toggle="wy-nav-shift" class="wy-nav-side">
<div class="wy-side-nav-search">
<a href="../index.html"> Python Anti-Patterns</a>
<div role="search">
<form id ="rtd-search-form" class="wy-form" action="../search.html" method="get">
<input type="text" name="q" placeholder="Search docs" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
</div>
</div>
<div class="wy-menu wy-menu-vertical" data-spy="affix" role="navigation" aria-label="main navigation">
<ul class="current">
<li class="toctree-l1 current"><a class="reference internal" href="index.html"><i class="fa fa-check"></i> Correctness</a><ul class="current">
<li class="toctree-l2"><a class="reference internal" href="accessing_a_protected_member_from_outside_the_class.html">Accessing a protected member from outside the class</a></li>
<li class="toctree-l2"><a class="reference internal" href="assigning_a_lambda_to_a_variable.html">Assigning a <cite>lambda</cite> expression to a variable</a></li>
<li class="toctree-l2"><a class="reference internal" href="assigning_to_builtin.html">Assigning to built-in function</a></li>
<li class="toctree-l2"><a class="reference internal" href="bad_except_clauses_order.html">Bad except clauses order</a></li>
<li class="toctree-l2"><a class="reference internal" href="bad_first_argument_given_to_super.html">Bad first argument given to <code class="docutils literal notranslate"><span class="pre">super()</span></code></a></li>
<li class="toctree-l2"><a class="reference internal" href="else_clause_on_loop_without_a_break_statement.html"><code class="docutils literal notranslate"><span class="pre">else</span></code> clause on loop without a <code class="docutils literal notranslate"><span class="pre">break</span></code> statement</a></li>
<li class="toctree-l2"><a class="reference internal" href="exit_must_accept_three_arguments.html"><code class="docutils literal notranslate"><span class="pre">__exit__</span></code> must accept 3 arguments: type, value, traceback</a></li>
<li class="toctree-l2"><a class="reference internal" href="explicit_return_in_init.html">Explicit return in __init__</a></li>
<li class="toctree-l2"><a class="reference internal" href="future_import_is_not_the_first_statement.html"><code class="docutils literal notranslate"><span class="pre">__future__</span></code> import is not the first non-docstring statement</a></li>
<li class="toctree-l2"><a class="reference internal" href="implementing_java-style_getters_and_setters.html">Implementing Java-style getters and setters</a></li>
<li class="toctree-l2"><a class="reference internal" href="indentation_contains_mixed_spaces_and_tabs.html">Indentation contains mixed spaces and tabs</a></li>
<li class="toctree-l2"><a class="reference internal" href="indentation_contains_tabs.html">Indentation contains tabs</a></li>
<li class="toctree-l2"><a class="reference internal" href="method_could_be_a_function.html">Method could be a function</a></li>
<li class="toctree-l2"><a class="reference internal" href="method_has_no_argument.html">Method has no argument</a></li>
<li class="toctree-l2"><a class="reference internal" href="missing_argument_to_super.html">Missing argument to <code class="docutils literal notranslate"><span class="pre">super()</span></code></a></li>
<li class="toctree-l2 current"><a class="current reference internal" href="#">Using a mutable default value as an argument</a></li>
<li class="toctree-l2"><a class="reference internal" href="no_exception_type_specified.html">No exception type(s) specified</a></li>
<li class="toctree-l2"><a class="reference internal" href="not_using_defaultdict.html">Not using <code class="docutils literal notranslate"><span class="pre">defaultdict()</span></code></a></li>
<li class="toctree-l2"><a class="reference internal" href="not_using_else_in_a_loop.html">Not using <code class="docutils literal notranslate"><span class="pre">else</span></code> where appropriate in a loop</a></li>
<li class="toctree-l2"><a class="reference internal" href="not_using_explicit_unpacking.html">Not using explicit unpacking</a></li>
<li class="toctree-l2"><a class="reference internal" href="not_using_get_to_return_a_default_value_from_a_dictionary.html">Not using <code class="docutils literal notranslate"><span class="pre">get()</span></code> to return a default value from a dict</a></li>
<li class="toctree-l2"><a class="reference internal" href="not_using_setdefault_to_initialize_a_dictionary.html">Not using <code class="docutils literal notranslate"><span class="pre">setdefault()</span></code> to initialize a dictionary</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="../maintainability/index.html"><i class="fa fa-puzzle-piece"></i> Maintainability</a></li>
<li class="toctree-l1"><a class="reference internal" href="../readability/index.html"><i class="fa fa-eye"></i> Readability</a></li>
<li class="toctree-l1"><a class="reference internal" href="../security/index.html"><i class="fa fa-lock"></i> Security</a></li>
<li class="toctree-l1"><a class="reference internal" href="../performance/index.html"><i class="fa fa-dashboard"></i> Performance</a></li>
<li class="toctree-l1"><a class="reference internal" href="../django/index.html"><i class="fa fa-book"></i> Django</a></li>
</ul>
</div>
</nav>
<section data-toggle="wy-nav-shift" class="wy-nav-content-wrap">
<nav class="wy-nav-top" role="navigation" aria-label="top navigation">
<i data-toggle="wy-nav-top" class="fa fa-bars"></i>
<a href="../index.html">Python Anti-Patterns</a>
</nav>
<div class="wy-nav-content" id="signup-box" >
<div class="rst-content">
<div class="navigation" role="navigation" aria-label="breadcrumbs navigation">
<ul class="wy-breadcrumbs">
<li><a href="../index.html">Documentation</a> »</li>
<li><a href="index.html"><i class="fa fa-check"></i> Correctness</a> »</li>
<li>Using a mutable default value as an argument</li>
<li class="wy-breadcrumbs-aside">
<a href="../_sources/correctness/mutable_default_value_as_argument.rst.txt" rel="nofollow"> View page source</a>
</li>
</ul>
<hr/>
</div>
<div role="main">
<div class="section" id="using-a-mutable-default-value-as-an-argument">
<h1>Using a mutable default value as an argument<a class="headerlink" href="#using-a-mutable-default-value-as-an-argument" title="Permalink to this headline">¶</a></h1>
<p>Passing mutable lists or dictionaries as default arguments to a function can have unforeseen consequences. Usually when a programmer uses a list or dictionary as the default argument to a function, the programmer wants the program to create a new list or dictionary every time that the function is called. However, this is not what Python does. The first time that the function is called, Python creates a persistent object for the list or dictionary. Every subsequent time the function is called, Python uses that same persistent object that was created from the first call to the function.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>A programmer wrote the <code class="docutils literal notranslate"><span class="pre">append</span></code> function below under the assumption that the <code class="docutils literal notranslate"><span class="pre">append</span></code> function would return a new list every time that the function is called without the second argument. In reality this is not what happens. The first time that the function is called, Python creates a persistent list. Every subsequent call to <code class="docutils literal notranslate"><span class="pre">append</span></code> appends the value to that original list.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">append</span><span class="p">(</span><span class="n">number</span><span class="p">,</span> <span class="n">number_list</span><span class="o">=</span><span class="p">[]):</span>
<span class="n">number_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">number</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">number_list</span><span class="p">)</span>
<span class="k">return</span> <span class="n">number_list</span>
<span class="n">append</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># expecting: [5], actual: [5]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span> <span class="c1"># expecting: [7], actual: [5, 7]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="c1"># expecting: [2], actual: [5, 7, 2]</span>
</pre></div>
</div>
</div>
<div class="section" id="best-practice">
<h2>Best practice<a class="headerlink" href="#best-practice" title="Permalink to this headline">¶</a></h2>
<div class="section" id="use-a-sentinel-value-to-denote-an-empty-list-or-dictionary">
<h3>Use a sentinel value to denote an empty list or dictionary<a class="headerlink" href="#use-a-sentinel-value-to-denote-an-empty-list-or-dictionary" title="Permalink to this headline">¶</a></h3>
<p>If, like the programmer who implemented the <code class="docutils literal notranslate"><span class="pre">append</span></code> function above, you want the function to return a new, empty list every time that the function is called, then you can use a <a class="reference external" href="http://en.wikipedia.org/wiki/Sentinel_value">sentinel value</a> to represent this use case, and then modify the body of the function to support this scenario. When the function receives the sentinel value, it knows that it is supposed to return a new list.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># the keyword None is the sentinel value representing empty list</span>
<span class="k">def</span> <span class="nf">append</span><span class="p">(</span><span class="n">number</span><span class="p">,</span> <span class="n">number_list</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="k">if</span> <span class="n">number_list</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">number_list</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">number_list</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">number</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">number_list</span><span class="p">)</span>
<span class="k">return</span> <span class="n">number_list</span>
<span class="n">append</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># expecting: [5], actual: [5]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">)</span> <span class="c1"># expecting: [7], actual: [7]</span>
<span class="n">append</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="c1"># expecting: [2], actual: [2]</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/dangerous-default-value.html">PyLint - W0102, dangerous-default-value</a></p></li>
<li><p><a class="reference external" href="http://stackoverflow.com/questions/101268/hidden-features-of-python#113198">Stack Overflow - Hidden Features of Python</a></p></li>
</ul>
</div>
</div>
</div>
<footer>
<div class="rst-footer-buttons" role="navigation" aria-label="footer navigation">
<a href="no_exception_type_specified.html" class="btn btn-neutral float-right" title="No exception type(s) specified"/>Next <span class="fa fa-arrow-circle-right"></span></a>
<a href="missing_argument_to_super.html" class="btn btn-neutral" title="Missing argument to super()"><span class="fa fa-arrow-circle-left"></span> Previous</a>
</div>
<hr/>
<div role="contentinfo">
<p>
</p>
</div>
<!--End mc_embed_signup-->
<a href="https://github.com/snide/sphinx_rtd_theme">Sphinx theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a> - Last updated: Sep 29, 2020
</footer>
</div>
</div>
</section>
</div>
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT:'../',
VERSION:'',
COLLAPSE_INDEX:false,
FILE_SUFFIX:'.html',
HAS_SOURCE: true
};
</script>
<script type="text/javascript" src="../_static/jquery.js"></script>
<script type="text/javascript" src="../_static/underscore.js"></script>
<script type="text/javascript" src="../_static/doctools.js"></script>
<script type="text/javascript" src="../_static/language_data.js"></script>
<script type="text/javascript" src="../_static/js/theme.js"></script>
<script type="text/javascript">
jQuery(function () {
SphinxRtdTheme.StickyNav.enable();
});
</script>
<!-- Place this tag right after the last button or just before your close body tag. -->
<script async defer id="github-bjs" src="https://buttons.github.io/buttons.js"></script>
</body>
</html>