Python doesn't know the meaning of life :-)

By Vasudev Ram

Attribution of above image

I was checking out the repr() and __repr__() functions / methods of Python (in the process of creating an exercise for one of my students), when I came across this interesting 'feature' of Python:

I did this at the Python interpreter prompt (in both 2.7 and 3.4):

>>> ''.__repr__() # Call the dunder repr method on an empty string.
"''"
>>>

(Got the expected output - the representation of the empty string.)

Similarly for a list, a tuple, etc.:

>>> [].__repr__()
'[]'


>>> ().__repr__()
'()'

But when I did this:

>>> 42.__repr__()

I got this:

File "<stdin>", line 1
42.__repr__()
^
SyntaxError: invalid syntax

So, it looks like Python does not know the meaning of life :-)

---

Jokes apart, I also tried this (actually, before trying 42):

>>> 1.0.__repr__()
'1.0'

which worked. And the only difference between the example above that worked (1.0), and the two that did not (1 or 42), is that the latter two use an int, and the former uses a number with a decimal point.

But both also use the dot as the attribute access operator, which leads me to think that this is the reason for the issue (for ints): there is an ambiguity between interpreting 1.something as 1.other_decimal_digits (making it a floating point number) and as 1.some_attribute (making it an attribute access on an object) (because 1, like (almost) everything in Python, is an object (of type int), as can be seen by doing:

>>> dir(1)
and
>>> type(1)

Interested to know if anyone has a better explanation of this, and also of whether the same issue/anomaly, if it is one, occurs in any other languages, or if not, why not.

- Vasudev Ram - Online Python training and programming

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Find the caller and caller's caller of a Python function

By Vasudev Ram

While browsing some Python posts on the Net, I saw one that made use of the sys._getframe() function. I had seen that function before, and remembered that it returns information about currently active stack frames. So I looked up the docs for sys._getframe(), thinking that it might be possible to use it within a function, to find that function's caller. Turned out that it could. I also googled for terms like "find the name of the calling function in python", and found two relevant Stack Overflow (SO) posts:

Getting the caller function name inside another function in Python?

Python: How to get the caller's method name in the called method?

It can be done by using either inspect.stack() or sys._getframe(). (The inspect module in Python's standard library supports that (finding a function's caller via stack frames), as well as some other useful introspection techniques.)

(I had blogged a couple of times about inspect, earlier, here:

Python's inspect module is powerful

and here:

Using inspect.getargvalues to debug Python programs

)

So I tried out the SO code examples and slightly modified them to make these two small programs that print the name of the caller, and the caller's caller.

Here is the first program:

'''
File: find_callers.py
Run with: python find_callers.py
'''

import sys

def foo():
    print "I am foo, calling bar:"
    bar()

def bar():
    print "I am bar, calling baz:"
    baz()

def baz():
    print "I am baz:"
    caller = sys._getframe(1).f_code.co_name
    callers_caller = sys._getframe(2).f_code.co_name
    print "I was called from", caller
    print caller, "was called from", callers_caller

foo()

When run, this program outputs:

I am foo, calling bar:
I am bar, calling baz:
I am baz:
I was called from bar
bar was called from foo

The second program is similar to the one above, except that, just for fun and to do it differently, I defined part of the overall code expression as a string, interpolated the argument (2 or 3) into it using Python string formatting, and then eval()'ed the resulting string. Note that since there is now an additional function call (eval), I had to change the arguments to sys._getframe() from 1 and 2 to 2 and 3:

# File: find_callers_with_eval.py
# Run with: python find_callers_with_eval.py

import sys

getframe_expr = 'sys._getframe({}).f_code.co_name'

def foo():
    print "I am foo, calling bar:"
    bar()

def bar():
    print "I am bar, calling baz:"
    baz()

def baz():
    print "I am baz:"
    caller = eval(getframe_expr.format(2))
    callers_caller = eval(getframe_expr.format(3))
    print "I was called from", caller
    print caller, "was called from", callers_caller

foo()

The output of this second program was identical to the first.

Note that the Python documentation says:

[ CPython implementation detail: This function should be used for internal and specialized purposes only. It is not guaranteed to exist in all implementations of Python. ]

Also, use eval() with care, and only on trusted code.

- Vasudev Ram - Online Python training and programming

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Python stdlib docs as Vim help files

By Vasudev Ram - dancingbison.com | @vasudevram | jugad2.blogspot.com

Saw this recently.

The Python standard library documentation as Vim help files:

http://jeetworks.org/node/92

It seems to be a work in progress but may be of use. The potential benefit is that you can look up Python library documentation from within Vim by typing ":help py2stdlib". Neat idea.

It is by Jeet Sukumaran ( http://jeetworks.org/about ) , currently a PhD student at the University of Kansas, who uses Python for his work.

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- Vasudev Ram @ Dancing Bison

Some ways of doing UNIX-style pipes in Python

By Vasudev Ram - dancingbison.com | @vasudevram | jugad2.blogspot.com

For a project I'm working on, I was recently thinking about how to implement something similar to UNIX-style pipes in Python; not necessarily exactly the same, but conceptually similar.

I had deliberately decided *not* to search for this on the Net, so that I could first think about it myself, and figure something out.
But coincidentally today, while browsing the Usenet group comp.lang,python, I came across this post mentioning issues with doing one-liners in Python.

One of the answers given was to check out PyP, a tool for Python that lets you do pipes (in a sense) and data munging like the powerful UNIX tools sed and awk. PyP stands for "Python Power at the Prompt, meaning the UNIX shell prompt, of course. It has an interesting and unusual approach. It is open source, hosted on Google Code, and was apparently initially created by a division of Sony Pictures called ImageWorks, "to facilitate the construction of complex image manipulation unix commands during visual effects work on Alice in Wonderland, Green Lantern, and the upcoming The Amazing Spiderman". Good performance was mentioned as one of it's plus points, apart from the pipe facility itself.

So I checked PyP out a bit and it seems like a nice tool. It has a fairly intuitive syntax for at least basic operations, and is also extensible in at least couple or so ways for more advanced users

I also did a Google query or two with appropriate keywords to find other such tools. Here are some of them, including PyP again:

PyP: http://code.google.com/p/pyp

Osh: http://geophile.com/osh/

Pipe module for Python by Julien Palard:
http://dev-tricks.net/pipe-infix-syntax-for-python

Piping support in the standard Python library:

http://docs.python.org/library/pipes.html

Will update this post later after checking these tools out some more.

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- Vasudev Ram @ Dancing Bison

Book: The Python Standard Library By Example, by Doug Hellmann

By Vasudev Ram - dancingbison.com | @vasudevram | jugad2.blogspot.com

Just got to know about this new Python book published by Addison-Wesley Professional:

The Python Standard Library By Example: by Doug Hellmann:

http://www.doughellmann.com/books/byexample/index.html

Excerpt about the book:

[ The Python Standard Library contains hundreds of modules that provide tools for interacting with the operating system, interpreter, and Internet—all of them tested and ready to be used to jump-start the development of your applications. This book presents examples demonstrating how to use the most commonly used features of the modules that give Python its "batteries included" slogan, taken from the popular Python Module of the Week (PyMOTW) blog series. ]

Doug Hellmann is a veteran Pythonista and the Communications Director of the Python Software Foundation, and is also heavily involved in Python in many other ways. I've read many of his blog posts from the PyMOTW series, and they are good.

The above web page has:

- a description of the book, and who the target audience is - the intermediate Python programmer - but even if you are a newbie Python programmer but experienced in other language(s), you can probably derive a lot of value from both the blog series and the book

- a link to a free PDF containing the table of contents, foreword, introduction, index, and chapter 2 ("Data Structures") from the final version of the manuscript

- a link to a free e-book containing the table of contents, foreword, introduction, index, and chapter 1 ("Text"), available through the iTunes book store

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- Vasudev Ram - dancingbison.com