psycopg2: insert multiple rows with one query

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I need to insert multiple rows with one query (number of rows is not constant), so I need to execute query like this one:

INSERT INTO t (a, b) VALUES (1, 2), (3, 4), (5, 6);

The only way I know is

args = [(1,2), (3,4), (5,6)]
args_str = ",".join(cursor.mogrify("%s", (x, )) for x in args)
cursor.execute("INSERT INTO t (a, b) VALUES "+args_str)

but I want some simpler way.

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psycopg2: insert multiple rows with one query insert: Questions

How to insert newlines on argparse help text?

5 answers

I"m using argparse in Python 2.7 for parsing input options. One of my options is a multiple choice. I want to make a list in its help text, e.g.

from argparse import ArgumentParser

parser = ArgumentParser(description="test")

parser.add_argument("-g", choices=["a", "b", "g", "d", "e"], default="a",
    help="Some option, where
         " a = alpha
         " b = beta
         " g = gamma
         " d = delta
         " e = epsilon")


However, argparse strips all newlines and consecutive spaces. The result looks like

~/Downloads:52$ python2.7 -h
usage: [-h] [-g {a,b,g,d,e}]


optional arguments:
  -h, --help      show this help message and exit
  -g {a,b,g,d,e}  Some option, where a = alpha b = beta g = gamma d = delta e
                  = epsilon

How to insert newlines in the help text?


Answer #1

Try using RawTextHelpFormatter:

from argparse import RawTextHelpFormatter
parser = ArgumentParser(description="test", formatter_class=RawTextHelpFormatter)

psycopg2: insert multiple rows with one query insert: Questions

Is a Python list guaranteed to have its elements stay in the order they are inserted in?

5 answers

If I have the following Python code

>>> x = []
>>> x = x + [1]
>>> x = x + [2]
>>> x = x + [3]
>>> x
[1, 2, 3]

Will x be guaranteed to always be [1,2,3], or are other orderings of the interim elements possible?


Answer #1

Yes, the order of elements in a python list is persistent.

psycopg2: insert multiple rows with one query insert: Questions

Inserting image into IPython notebook markdown

5 answers

I am starting to depend heavily on the IPython notebook app to develop and document algorithms. It is awesome; but there is something that seems like it should be possible, but I can"t figure out how to do it:

I would like to insert a local image into my (local) IPython notebook markdown to aid in documenting an algorithm. I know enough to add something like <img src="image.png"> to the markdown, but that is about as far as my knowledge goes. I assume I could put the image in the directory represented by (or some subdirectory) to be able to access it, but I can"t figure out where that directory is. (I"m working on a mac.) So, is it possible to do what I"m trying to do without too much trouble?


Answer #1

Most of the answers given so far go in the wrong direction, suggesting to load additional libraries and use the code instead of markup. In Ipython/Jupyter Notebooks it is very simple. Make sure the cell is indeed in markup and to display a image use:

![alt text](imagename.png "Title")

Further advantage compared to the other methods proposed is that you can display all common file formats including jpg, png, and gif (animations).

Why is it string.join(list) instead of list.join(string)?

5 answers

Evan Fosmark By Evan Fosmark

This has always confused me. It seems like this would be nicer:

my_list = ["Hello", "world"]
# Produce: "Hello-world"

Than this:

my_list = ["Hello", "world"]
# Produce: "Hello-world"

Is there a specific reason it is like this?


Answer #1

It"s because any iterable can be joined (e.g, list, tuple, dict, set), but its contents and the "joiner" must be strings.

For example:

"_".join(["welcome", "to", "stack", "overflow"])
"_".join(("welcome", "to", "stack", "overflow"))

Using something other than strings will raise the following error:

TypeError: sequence item 0: expected str instance, int found


Answer #2

This was discussed in the String methods... finally thread in the Python-Dev achive, and was accepted by Guido. This thread began in Jun 1999, and str.join was included in Python 1.6 which was released in Sep 2000 (and supported Unicode). Python 2.0 (supported str methods including join) was released in Oct 2000.

  • There were four options proposed in this thread:
    • str.join(seq)
    • seq.join(str)
    • seq.reduce(str)
    • join as a built-in function
  • Guido wanted to support not only lists and tuples, but all sequences/iterables.
  • seq.reduce(str) is difficult for newcomers.
  • seq.join(str) introduces unexpected dependency from sequences to str/unicode.
  • join() as a built-in function would support only specific data types. So using a built-in namespace is not good. If join() supports many datatypes, creating an optimized implementation would be difficult, if implemented using the __add__ method then it would ve O(n¬≤).
  • The separator string (sep) should not be omitted. Explicit is better than implicit.

Here are some additional thoughts (my own, and my friend"s):

  • Unicode support was coming, but it was not final. At that time UTF-8 was the most likely about to replace UCS2/4. To calculate total buffer length of UTF-8 strings it needs to know character coding rule.
  • At that time, Python had already decided on a common sequence interface rule where a user could create a sequence-like (iterable) class. But Python didn"t support extending built-in types until 2.2. At that time it was difficult to provide basic iterable class (which is mentioned in another comment).

Guido"s decision is recorded in a historical mail, deciding on str.join(seq):

Funny, but it does seem right! Barry, go for it...
Guido van Rossum


Answer #3

Because the join() method is in the string class, instead of the list class?

I agree it looks funny.


Historical note. When I first learned Python, I expected join to be a method of a list, which would take the delimiter as an argument. Lots of people feel the same way, and there’s a story behind the join method. Prior to Python 1.6, strings didn’t have all these useful methods. There was a separate string module which contained all the string functions; each function took a string as its first argument. The functions were deemed important enough to put onto the strings themselves, which made sense for functions like lower, upper, and split. But many hard-core Python programmers objected to the new join method, arguing that it should be a method of the list instead, or that it shouldn’t move at all but simply stay a part of the old string module (which still has lots of useful stuff in it). I use the new join method exclusively, but you will see code written either way, and if it really bothers you, you can use the old string.join function instead.

--- Mark Pilgrim, Dive into Python


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