I am moving some of my code onto sqlalchemy from using raw MySQL queries.
The current issue I am having is that the datetime was saved in a string format by a C# tool. Unfortunately, the representation does not match up with Python's (as well as that it has an extra set of single quotes), thus making filtering somewhat cumbersome.
Here is an example of the format that the date was saved in:
'2016-07-01T17:27:01'
Which I was able to convert to a usable datetime using the following MySQL command:
STR_TO_DATE(T.PredicationGeneratedTime, \"'%%Y-%%m-%%dT%%H:%%i:%%s'\")
However, I cannot find any documentation that describes how to invoke built-in functions such as STR_TO_DATE when filtering with sqlalchemy
The following Python code:
session.query(Train.Model).filter(cast(Train.Model.PredicationGeneratedTime, date) < start)
is giving me:
TypeError: Required argument 'year' (pos 1) not found
There does not seem to be a way to specify the format for the conversion.
Note: I realize the solution is to fix the way the datetime is stored, but in the mean time I'd like to run queries against the existing data.
You can try to use func.str_to_date(COLUMN, FORMAT_STRING) instead of cast
In the cast() you should be using sqlalchemy.DateTime, not (what I assume is) a datetime.date - that is the cause of the exception.
However, fixing that will not really help because of the embedded single quotes.
You are fortunate that the dates stored in your table are in ISO format. That means that lexicographic comparisons will work on the date strings themselves, without casting. As long as you use a string for start with the surrounding single quotes, it will work.
from datetime import datetime
start = "'{}'".format(datetime.now().isoformat())
session.query(Train.Model).filter(Train.Model.PredicationGeneratedTime < start)
I have chapter times in the form of HH:MM:SS. I am parsing them from a document, and I will have times as a string in the format of '00:12:14'. How would I store this in a mysql column, and then retrieve it in the required format to be able to:
1) order by time;
2) convert to a string in the above format.
I suggest you look at the MySQL time type. It will allow you to sort and format as you wish.
http://dev.mysql.com/doc/refman/5.0/en/time.html
Use the TIME type.
It allows "time values to be represented in several formats, such as quoted strings or as numbers, depending on the exact type of the value and other factors." In addition, you can perform various functions to manipulate the time.
If I have such a simple task, I choose a simple solution: I would choose the python datetime.time module (see: datetime.time) and store a TIME object using strftime.
Loading it back in is a little painful as you would have to split your string at : and then pass the values to the time constructor. Example:
def load(timestr):
hours,minutes,seconds = timestr.split(":")
return datetime.time(hours,minutes,seconds)
Hope this helps.
With PostgreSQL, one of my tables has an 'interval' column, values of which I would like to extract as something I can manipulate (datetime.timedelta?); however I am using PyGreSQL which seems to be returning intervals as strings, which is less than helpful.
Where should I be looking to either parse the interval or make PyGreSQL return it as a <something useful>?
Use Psycopg 2. It correctly converts between Postgres's interval data type and Python's timedelta.
In one of my Django projects that use MySQL as the database, I need to have a date fields that accept also "partial" dates like only year (YYYY) and year and month (YYYY-MM) plus normal date (YYYY-MM-DD).
The date field in MySQL can deal with that by accepting 00 for the month and the day. So 2010-00-00 is valid in MySQL and it represent 2010. Same thing for 2010-05-00 that represent May 2010.
So I started to create a PartialDateField to support this feature. But I hit a wall because, by default, and Django use the default, MySQLdb, the python driver to MySQL, return a datetime.date object for a date field AND datetime.date() support only real date. So it's possible to modify the converter for the date field used by MySQLdb and return only a string in this format 'YYYY-MM-DD'. Unfortunately the converter use by MySQLdb is set at the connection level so it's use for all MySQL date fields. But Django DateField rely on the fact that the database return a datetime.date object, so if I change the converter to return a string, Django is not happy at all.
Someone have an idea or advice to solve this problem? How to create a PartialDateField in Django ?
EDIT
Also I should add that I already thought of 2 solutions, create 3 integer fields for year, month and day (as mention by Alison R.) or use a varchar field to keep date as string in this format YYYY-MM-DD.
But in both solutions, if I'm not wrong, I will loose the special properties of a date field like doing query of this kind on them: Get all entries after this date. I can probably re-implement this functionality on the client side but that will not be a valid solution in my case because the database can be query from other systems (mysql client, MS Access, etc.)
First, thanks for all your answers. None of them, as is, was a good solution for my problem, but, for your defense, I should add that I didn't give all the requirements. But each one help me think about my problem and some of your ideas are part of my final solution.
So my final solution, on the DB side, is to use a varchar field (limited to 10 chars) and storing the date in it, as a string, in the ISO format (YYYY-MM-DD) with 00 for month and day when there's no month and/or day (like a date field in MySQL). This way, this field can work with any databases, the data can be read, understand and edited directly and easily by a human using a simple client (like mysql client, phpmyadmin, etc.). That was a requirement. It can also be exported to Excel/CSV without any conversion, etc. The disadvantage is that the format is not enforce (except in Django). Someone could write 'not a date' or do a mistake in the format and the DB will accept it (if you have an idea about this problem...).
This way it's also possible to do all of the special queries of a date field relatively easily. For queries with WHERE: <, >, <=, >= and = work directly. The IN and BETWEEN queries work directly also. For querying by day or month you just have to do it with EXTRACT (DAY|MONTH ...). Ordering work also directly. So I think it covers all the query needs and with mostly no complication.
On the Django side, I did 2 things. First, I have created a PartialDate object that look mostly like datetime.date but supporting date without month and/or day. Inside this object I use a datetime.datetime object to keep the date. I'm using the hours and minutes as flag that tell if the month and day are valid when they are set to 1. It's the same idea that steveha propose but with a different implementation (and only on the client side). Using a datetime.datetime object gives me a lot of nice features for working with dates (validation, comparaison, etc.).
Secondly, I have created a PartialDateField that mostly deal with the conversion between the PartialDate object and the database.
So far, it works pretty well (I have mostly finish my extensive unit tests).
You could store the partial date as an integer (preferably in a field named for the portion of the date you are storing, such as year, month or day) and do validation and conversion to a date object in the model.
EDIT
If you need real date functionality, you probably need real, not partial, dates. For instance, does "get everything after 2010-0-0" return dates inclusive of 2010 or only dates in 2011 and beyond? The same goes for your other example of May 2010. The ways in which different languages/clients deal with partial dates (if they support them at all) are likely to be highly idiosyncratic, and they are unlikely to match MySQL's implementation.
On the other hand, if you store a year integer such as 2010, it is easy to ask the database for "all records with year > 2010" and understand exactly what the result should be, from any client, on any platform. You can even combine this approach for more complicated dates/queries, such as "all records with year > 2010 AND month > 5".
SECOND EDIT
Your only other (and perhaps best) option is to store truly valid dates and come up with a convention in your application for what they mean. A DATETIME field named like date_month could have a value of 2010-05-01, but you would treat that as representing all dates in May, 2010. You would need to accommodate this when programming. If you had date_month in Python as a datetime object, you would need to call a function like date_month.end_of_month() to query dates following that month. (That is pseudocode, but could be easily implemented with something like the calendar module.)
It sounds like you want to store a date interval. In Python this would (to my still-somewhat-noob understanding) most readily be implemented by storing two datetime.datetime objects, one specifying the start of the date range and the other specifying the end. In a manner similar to that used to specify list slices, the endpoint would not itself be included in the date range.
For example, this code would implement a date range as a named tuple:
>>> from datetime import datetime
>>> from collections import namedtuple
>>> DateRange = namedtuple('DateRange', 'start end')
>>> the_year_2010 = DateRange(datetime(2010, 1, 1), datetime(2011, 1, 1))
>>> the_year_2010.start <= datetime(2010, 4, 20) < the_year_2010.end
True
>>> the_year_2010.start <= datetime(2009, 12, 31) < the_year_2010.end
False
>>> the_year_2010.start <= datetime(2011, 1, 1) < the_year_2010.end
False
Or even add some magic:
>>> DateRange.__contains__ = lambda self, x: self.start <= x < self.end
>>> datetime(2010, 4, 20) in the_year_2010
True
>>> datetime(2011, 4, 20) in the_year_2010
False
This is such a useful concept that I'm pretty sure that somebody has already made an implementation available. For example, a quick glance suggests that the relativedate class from the dateutil package will do this, and more expressively, by allowing a 'years' keyword argument to be passed to the constructor.
However, mapping such an object into database fields is somewhat more complicated, so you might be better off implementing it simply by just pulling both fields separately and then combining them. I guess this depends on the DB framework; I'm not very familiar with that aspect of Python yet.
In any case, I think the key is to think of a "partial date" as a range rather than as a simple value.
edit
It's tempting, but I think inappropriate, to add more magic methods that will handle uses of the > and < operators. There's a bit of ambiguity there: does a date that's "greater than" a given range occur after the range's end, or after its beginning? It initially seems appropriate to use <= to indicate that the date on the right-hand side of the equation is after the start of the range, and < to indicate that it's after the end.
However, this implies equality between the range and a date within the range, which is incorrect, since it implies that the month of May, 2010 is equal to the year 2010, because May the 4th, 2010 equates to the both of them. IE you would end up with falsisms like 2010-04-20 == 2010 == 2010-05-04 being true.
So probably it would be better to implement a method like isafterstart to explicitly check if a date is after the beginning of the range. But again, somebody's probably already done it, so it's probably worth a look on pypi to see what's considered production-ready. This is indicated by the presence of "Development Status :: 5 - Production/Stable" in the "Categories" section of a given module's pypi page. Note that not all modules have been given a development status.
Or you could just keep it simple, and using the basic namedtuple implementation, explicitly check
>>> datetime(2012, 12, 21) >= the_year_2010.start
True
Can you store the date together with a flag that tells how much of the date is valid?
Something like this:
YEAR_VALID = 0x04
MONTH_VALID = 0x02
DAY_VALID = 0x01
Y_VALID = YEAR_VALID
YM_VALID = YEAR_VALID | MONTH_VALID
YMD_VALID = YEAR_VALID | MONTH_VALID | DAY_VALID
Then, if you have a date like 2010-00-00, convert that to 2010-01-01 and set the flag to Y_VALID. If you have a date like 2010-06-00, convert that to 2010-06-01 and set the flag to YM_VALID.
So, then, PartialDateField would be a class that bundles together a date and the date-valid flag described above.
P.S. You don't actually need to use the flags the way I showed it; that's the old C programmer in me coming to the surface. You could use Y_VALID, YM_VALID, YMD_VALID = range(3) and it would work about as well. The key is to have some kind of flag that tells you how much of the date to trust.
Although not in Python - here's an example of how the same problem was solved in Ruby - using a single Integer value - and bitwise operators to store year, month and day - with month and day optional.
https://github.com/58bits/partial-date
Look at the source in lib for date.rb and bits.rb.
I'm sure a similar solution could be written in Python.
To persist the date (sortable) you just save the Integer to the database.
SQLite docs specifies that the preferred format for storing datetime values in the DB is to use Julian Day (using built-in functions).
However, all frameworks I saw in python (pysqlite, SQLAlchemy) store the datetime.datetime values as ISO formatted strings. Why are they doing so?
I'm usually trying to adapt the frameworks to storing datetime as julianday, and it's quite painful. I started to doubt that is worth the efforts.
Please share your experience in this field with me. Does sticking with julianday make sense?
Julian Day is handy for all sorts of date calculations, but it can's store the time part decently (with precise hours, minutes, and seconds). In the past I've used both Julian Day fields (for dates), and seconds-from-the-Epoch (for datetime instances), but only when I had specific needs for computation (of dates and respectively of times). The simplicity of ISO formatted dates and datetimes, I think, should make them the preferred choice, say about 97% of the time.
Store it both ways. Frameworks can be set in their ways and if yours is expecting to find a raw column with an ISO formatted string then that is probably more of a pain to get around than it's worth.
The concern in having two columns is data consistency but sqlite should have everything you need to make it work. Version 3.3 has support for check constraints and triggers. Read up on date and time functions. You should be able to do what you need entirely in the database.
CREATE TABLE Table1 (jd, isotime);
CREATE TRIGGER trigger_name_1 AFTER INSERT ON Table1
BEGIN
UPDATE Table1 SET jd = julianday(isotime) WHERE rowid = last_insert_rowid();
END;
CREATE TRIGGER trigger_name_2 AFTER UPDATE OF isotime ON Table1
BEGIN
UPDATE Table1 SET jd = julianday(isotime) WHERE rowid = old.rowid;
END;
And if you cant do what you need within the DB you can write a C extension to perform the functionality you need. That way you wont need to touch the framework other than to load your extension.
But typically, the Human doesn't read directly from the database. Fractional time on a Julian Day is easily converted to human readible by (for example)
void hour_time(GenericDate *ConvertObject)
{
double frac_time = ConvertObject->jd;
double hour = (24.0*(frac_time - (int)frac_time));
double minute = 60.0*(hour - (int)hour);
double second = 60.0*(minute - (int)minute);
double microsecond = 1000000.0*(second - (int)second);
ConvertObject->hour = hour;
ConvertObject->minute = minute;
ConvertObject->second = second;
ConvertObject->microsecond = microsecond;
};
Because 2010-06-22 00:45:56 is far easier for a human to read than 2455369.5318981484. Text dates are great for doing ad-hoc queries in SQLiteSpy or SQLite Manager.
The main drawback, of course, is that text dates require 19 bytes instead of 8.