Category Archives: Structured Data

When literal_eval fails: using asterisk notation to read in a datetime object

One great function in python is the ast  (Abstract Syntax Tree) library’s literal_eval . This lets you read in a string version of a python datatype:

Importing a dictionary such as this is similar to parsing JSON using Python’s  json.loads decoder. But it also comes with the shortcoming’s of JSON’s restrictive datatypes, as we can see here when the dictionary contains, for example, a datetime object:

So you might try and write some code to parse the dictionary data-type yourself. This gets very tricky, but eventually you could probably accommodate for all common data-types:

But this still doesn’t truly fix our datetime object problem:

Which is where we get to the crux of this post. I thought at first that I could deal with datetime’s formatting by extracting the class  datetime.datetime(2013, 8, 10, 21, 46, 52, 638649) as a tuple by spotting the brackets, then feeding the tuple back into datetime like: 

But apparently not. The tuple must be extracted – not by a lambda or perhaps list comprehension, but in fact by using asterisk notation:

Asterisk ( * ) unpacks an iterable such as x into positional arguments for the function. Simple!