There's no fumbling around at all now. In this case, the default factory function is list , which returns an empty list. This is how to get a dictionary with default values of 0: use int as a default factory function:.
In Python 3. For compatibility, add this to the class definition:. Say we want to iterate over the items, and we need both the item's index and the item itself:. The enumerate function takes a list and returns index, item pairs:. The enumerate version is much shorter and simpler than the version on the left, and much easier to read and understand than either.
An example showing how the enumerate function actually returns an iterator a generator is a kind of iterator :. The original integer 1 object no longer has a tag "a". It may live on, but we can't get to it through the name "a". When an object has no more references or tags, it is removed from memory.
Although we commonly refer to "variables" even in Python because it's common terminology , we really mean "names" or "identifiers". In Python, "variables" are nametags for values, not labelled boxes. If you get nothing else out of this tutorial, I hope you understand how Python names work. A good understanding is certain to pay dividends, helping you to avoid cases like this:. Although if you don't know C, that's not very helpful.
Basically, you provide a template or format and interpolation values. Here we specify the names of interpolation values, which are looked up in the supplied dictionary.
Notice any redundancy? The names "name" and "messages" are already defined in the local namespace. We can take advantage of this. The locals function returns a dictionary of all locally-available names. This is very powerful. With this, you can do all the string formatting you want without having to worry about matching the interpolation values to the template.
But power can be dangerous. This is just something to keep in mind. The traditional way, with for and if statements:. Generator expressions "genexps" are just like list comprehensions, except that where listcomps are greedy, generator expressions are lazy. Listcomps compute the entire result list all at once, as a list.
Generator expressions compute one value at a time, when needed, as individual values. This is especially useful for long sequences where the computed list is just an intermediate step and not the final result. In this case, we're only interested in the sum; we don't need the intermediate list of squares.
We use xrange for the same reason: it lazily produces values, one at a time. We needed a dictionary mapping month numbers both as string and as integers to month codes for futures contracts. It can be done in one logical line of code. Note that the list is sorted in-place: the original list is sorted, and the sort method does not return the list or a copy. But what if you have a list of data that you need to sort, but it doesn't sort naturally i. You may need to sort on the second column first, then the fourth column.
We can use list's built-in sort method with a custom function:. See the next section, Sorting With Keys for the new approach.
The first line creates a list containing tuples: copies of the sort terms in priority order, followed by the complete data record. The third line retrieves the last value from the sorted list. Remember, this last value is the complete data record. We're throwing away the sort terms, which have done their job and are no longer needed. You can make your own key function, or use any existing one-argument function if applicable:.
The yield keyword turns a function into a generator. When you call a generator function, instead of running the code immediately Python returns a generator object, which is an iterator; it has a next method.
You can raise StopIteration explicitly, or implicitly by falling off the end of the generator code as above. The generator function maintains state. This is how a for loop really works. Python looks at the sequence supplied after the in keyword. If it's a simple container such as a list, tuple, dictionary, set, or user-defined container Python converts it into an iterator. If it's already an iterator, Python uses it directly.
Then Python repeatedly calls the iterator's next method, assigns the return value to the loop counter i in this case , and executes the indented code. This is repeated over and over, until StopIteration is raised, or a break statement is executed in the code. A for loop can have an else clause, whose code is executed after the iterator runs dry, but not after a break statement is executed. This distinction allows for some elegant uses.
Sometimes an else clause perfectly expresses the logic you need. This is possible because files support a next method, as do other iterators: lists, tuples, dictionaries for their keys , generators. There is a caveat here: because of the way the buffering is done, you cannot mix.
If it walks like a duck, and talks like a duck, and looks like a duck: it's a duck. Close enough. Use coercion if an object must be a particular type. If x must be a string for your code to work, why not call. You've probably seen this "wild card" form of the import statement. You may even like it. Don't use it. To adapt a well-known exchange :. You'll get things in your local namespace that you didn't expect to get. You may see imported names obscuring module-defined local names.
You won't be able to figure out where certain names come from. Although a convenient shortcut, this should not be in production code. So the code guarded by the if statement above will not run when imported. Except for special cases, you shouldn't put any major executable code at the top-level. Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. My credentials: I am a resident of Montreal, father of two great kids, husband of one special woman, a full-time Python programmer, author of the Docutils project and reStructuredText , an editor of the Python Enhancement Proposals or PEPs , an organizer of PyCon , and chair of PyCon , a member of the Python Software Foundation, a Director of the Foundation for the past year, and its Secretary.
The Zen of Python 1 These are the guiding principles of Python, but are open to interpretation. The Zen of Python 2 In the face of ambiguity, refuse the temptation to guess. Coding Style: Readability Counts Programs must be written for people to read, and only incidentally for machines to execute. Whitespace 1 4 spaces per indentation level.
No hard tabs. Never mix tabs and spaces. No spaces just inside parentheses or just before argument lists. No spaces just inside docstrings. If you add a space after the backslash, it won't work any more. Also, they're ugly. Long Strings Adjacent literal strings are concatenated by the parser:.
The parentheses allow implicit line continuation. Multiline strings use triple quotes:. In the last example above triple single quotes , note how the backslashes are used to escape the newlines. This eliminates extra newlines, while keeping the text and quotes nicely left-justified. I had been looking for a way to write text notes on the iPad and convert them into PDF files without the need for a commercial app or a network connection I often write lead sheets for my band during train rides , but all the solutions I found either required buying another app, just for the purpose of PDF export or to use a web service, which doesn't really work well on-the-go.
Anyway, to cut a long story short: with Pythonista and some external pure-Python packages I was able to write my own solution in no time. Here it is: MarkdownPDF. The script contains some documentation in its docstring in Markdown format, naturally. To use it, you need to install the required Python packages. I have created another small script to download a Zip archive I created, which bundles all the required packages, and extracts them to the 'site-packages' sub-directory:.
Then run the MarkdownPDF. You will be able to either view the documentation, convert Markdown from the clipboard or from a URL. After the conversion, you will be asked whether you want to save the PDF output to a file, open it in another app, or view it with console.
Thank you! I've been exploring different methods of pdf creation for a work application. I never considered markdown to pdf. I'm currently trying to "port" this to Editorial, so far without success.
The script downloads the required packages but the site-packages folder doesn't get created. Maybe this is not possible at all, but if someone knows a solution, that would be great! I don't have or know Editorial, so I don't know how installation of third-party Python packages works for this app.
Maybe scripts in Editorial are run in a different directory than in Pythonista, i. If that doesn't work, you can always download the markdown2pdf. Germany, September Elsevier Books Reference. Elsevier Books Reference. Related Audiobooks Free with a 30 day trial from Scribd. Elizabeth Howell. Views Total views. Actions Shares. No notes for slide. Code Like Pythonista 1. Explicit is better than implicit. Simple is better than complex. Complex is better than complicated. Flat is better than nested.
Sparse is better than dense. Readability counts. Special cases aren't special enough to break the rules. Although practicality beats purity. Errors should never pass silently. Unless explicitly silenced. In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it. Although that way may not be obvious at first unless you're Dutch. Now is better than never. If the implementation is hard to explain, it's a bad idea. If the implementation is easy to explain, it may be a good idea. Namespaces are one honking great idea -- let's do more of those!
Style Guide For Python Code 5. I never use it. Trust me.
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