How to justify columns using format function? - python-3.x

Question

I have a working function that takes a list made up of lists and outputs it as a table. I am just missing certain spacing and new lines. I'm pretty new to formatting strings (and python in general.) How do I use the format function to fix my output?
For examples:
>>> show_table([['A','BB'],['C','DD']])
'| A | BB |\n| C | DD |\n'
>>> print(show_table([['A','BB'],['C','DD']]))
| A | BB |
| C | DD |
>>> show_table([['A','BBB','C'],['1','22','3333']])
'| A | BBB | C |\n| 1 | 22 | 3333 |\n'
>>> print(show_table([['A','BBB','C'],['1','22','3333']]))
| A | BBB | C |
| 1 | 22 | 3333 |
What I am actually outputting though:
>>>show_table([['A','BB'],['C','DD']])
'| A | BB | C | DD |\n'
>>>show_table([['A','BBB','C'],['1','22','3333']])
'| A | BBB | C | 1 | 22 | 3333 |\n'
>>>show_table([['A','BBB','C'],['1','22','3333']])
| A | BBB | C | 1 | 22 | 3333 |
I will definitely need to use the format function but I'm not sure how?
This is my current code (my indenting is actually correct but I'm horrible with stackoverflow format):
def show_table(table):
if table is None:
table=[]
new_table = ""
for row in table:
for val in row:
new_table += ("| " + val + " ")
new_table += "|\n"
return new_table

Answer 1

You do actually have an indentation error in your function: the line
new_table += "|\n"
should be indented further so that it happens at the end of each row, not at the end of the table.
Side note: you'll catch this kind of thing more easily if you stick to 4 spaces per indent. This and other conventions are there to help you, and it's a very good idea to learn the discipline of keeping to them early in your progress with Python. PEP 8 is a great resource to familarise yourself with.
The spacing on your "what I need" examples is also rather messed up, which is unfortunate since spacing is the subject of your question, but I gather from this question that you want each column to be properly aligned, e.g.
>>> print(show_table([['10','2','300'],['4000','50','60'],['7','800','90000']]))
| 10 | 2 | 300 |
| 4000 | 50 | 60 |
| 7 | 800 | 90000 |
In order to do that, you'll need to know in advance what the maximum width of each item in a column is. That's actually a little tricky, because your table is organised into rows rather than columns, but the zip() function can help. Here's an example of what zip() does:
>>> table = [['10', '2', '300'], ['4000', '50', '60'], ['7', '800', '90000']]
>>> from pprint import pprint
>>> pprint(table, width=30)
[['10', '2', '300'],
['4000', '50', '60'],
['7', '800', '90000']]
>>> flipped = zip(*table)
>>> pprint(flipped, width=30)
[('10', '4000', '7'),
('2', '50', '800'),
('300', '60', '90000')]
As you can see, zip() turns rows into columns and vice versa. (don't worry too much about the * before table right now; it's a bit advanced to explain for the moment. Just remember that you need it).
You get the length of a string with len():
>>> len('800')
3
You get the maximum of the items in a list with max():
>>> max([2, 4, 1])
4
You can put all these together in a list comprehension, which is like a compact for loop that builds a list:
>>> widths = [max([len(x) for x in col]) for col in zip(*table)]
>>> widths
[4, 3, 5]
If you look carefully, you'll see that there are actually two list comprehensions in that line:
[len(x) for x in col]
makes a list with the lengths of each item x in a list col, and
[max(something) for col in zip(*table)]
makes a list with the maximum value of something for each column in the flipped (with zip) table … where something is the other list comprehension.
That's all kinda complicated the first time you see it, so spend a little while making sure you understand what's going on.
Now that you have your maximum widths for each column, you can use them to format your output. In order to do so, though, you need to keep track of which column you're in, and to do that, you need enumerate(). Here's an example of enumerate() in action:
>>> for i, x in enumerate(['a', 'b', 'c']):
... print("i is", i, "and x is", x)
...
i is 0 and x is a
i is 1 and x is b
i is 2 and x is c
As you can see, iterating over the result of enumerate() gives you two values: the position in the list, and the item itself.
Still with me? Fun, isn't it? Pressing on ...
The only thing left is the actual formatting. Python's str.format() method is pretty powerful, and too complex to explain thoroughly in this answer. One of the things you can use it for is to pad things out to a given width:
>>> "{val:5s}".format(val='x')
'x '
In the example above, {val:5s} says "insert the value of val here as a string, padding it out to 5 spaces". You can also specify the width as a variable, like this:
>>> "{val:{width}s}".format(val='x', width=3)
'x '
These are all the pieces you need … and here's a function that uses all those pieces:
def show_table(table):
if table is None:
table = []
new_table = ""
widths = [max([len(x) for x in c]) for c in zip(*table)]
for row in table:
for i, val in enumerate(row):
new_table += "| {val:{width}s} ".format(val=val, width=widths[i])
new_table += "|\n"
return new_table
… and here it is in action:
>>> table = [['10','2','300'],['4000','50','60'],['7','800','90000']]
>>> print(show_table(table))
| 10 | 2 | 300 |
| 4000 | 50 | 60 |
| 7 | 800 | 90000 |
I've covered a fair bit of ground in this answer. Hopefully if you study the final version of show_table() given here in detail (as well as the docs linked to throughout the answer), you'll be able to see how all the pieces described earlier on fit together.