Use variables as keywords for functions with pre-defined keywords (Python)
<p>I'm trying to create a simple function that allows a user to input basic information that will change the values of a datetime object, and I'd like to find a way to make it as clean as possible by using a variable as a keyword.  This can easily be done a different way, but I figured it'd be useful to know how to replace pre-set keywords.</p>

<p>The datetime object has a .replace() method that takes any time value as a keyword:</p>

<blockquote>
  <p>datetime.replace([year[, month[, day[, hour[, minute[, second[, microsecond[, tzinfo]]]]]]]])</p>
</blockquote>

<p>But I want to allow the user to specify how much of which kind of time (e.g., 2 days; 4 hours; 1 month).</p>

<p>I'm trying to replace any of the above keywords up to "minute" with whatever the user inputs, which is stored in the <em>time_type</em> variable, but I get "<strong>TypeError: 'time_type' is an invalid keyword argument for this function</strong>".</p>

<pre><code>    start_time = datetime.datetime(2016, 09, 16, 15, 30)

    def change_time(integer, time_string):
        time_type = time_string.replace("s","")  # de-pluralizes input
        new_time = getattr(start_time, time_type) + integer
        print(start_time.replace(time_type=new_time))

    change_time(2, "days")
</code></pre>

<p>This should print the new start_time, which is (2016, 09, 18, 15, 30), but I just get an error.</p>

<p>Python allows you to store data in a dictionary and finally unpack them as keyword arguments to different functions.</p>

<p>For the thing that you want to do, best way to accomplish this is to use kwargs.</p>

<pre><code>replacement_info = {'day': 2, 'month': 9, ...}

new_time = start_time.replace(**replacement_info)
</code></pre>

<p>Note the difference with what you've done. Passing <code>time_type</code> directly to <code>replace</code>, would result in <code>replace</code> being called with the <code>time_type</code> parameter, set to 2, which is undefined (since it's not in the list of accepted arguments for replace)</p>

<p>Instead you have to pass it like <code>**{time_type: 2}</code> to the replace function, this way, replace will receive the interpreted value for time_type, namely day, as the input.</p>

<p>So you need to change </p>

<pre><code>print(start_time.replace(time_type=new_time))
</code></pre>

<p>to </p>

<pre><code>print(start_time.replace(**{time_type:new_time})
</code></pre>

python datetime