Changeset 807 for trunk/docs
 Timestamp:
 Feb 1, 2016, 4:28:12 AM (4 years ago)
 Location:
 trunk/docs/contents
 Files:

 2 edited
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trunk/docs/contents/pg/adaptation.rst
r799 r807 299 299 :meth:`set_typecast`, you may need to run ``db.dbtypes.reset_typecast()`` 300 300 to make these changes effective on connections that were already open. 301 302 As one last example, let us try to typecast the geometric data type ``circle`` 303 of PostgreSQL into a `SymPy <http://www.sympy.org>`_ ``Circle`` object. Let's 304 assume we have created and populated a table with two circles, like so: 305 306 .. codeblock:: sql 307 308 CREATE TABLE circle ( 309 name varchar(8) primary key, circle circle); 310 INSERT INTO circle VALUES ('C1', '<(2, 3), 3>'); 311 INSERT INTO circle VALUES ('C2', '<(1, 1), 4>'); 312 313 With PostgreSQL we can easily calculate that these two circles overlap:: 314 315 >>> q = db.query("""SELECT c1.circle && c2.circle 316 ... FROM circle c1, circle c2 317 ... WHERE c1.name = 'C1' AND c2.name = 'C2'""") 318 >>> q.getresult()[0][0] 319 True 320 321 However, calculating the intersection points between the two circles using the 322 ``#`` operator does not work (at least not as of PostgreSQL version 9.5). 323 So let' resort to SymPy to find out. To ease importing circles from 324 PostgreSQL to SymPy, we create and register the following typecast function:: 325 326 >>> from sympy import Point, Circle 327 >>> 328 >>> def cast_circle(s): 329 ... p, r = s[1:1].split(',') 330 ... p = p[1:1].split(',') 331 ... return Circle(Point(float(p[0]), float(p[1])), float(r)) 332 ... 333 >>> pg.set_typecast('circle', cast_circle) 334 335 Now we can import the circles in the table into Python simply using:: 336 337 >>> circle = db.get_as_dict('circle', scalar=True) 338 339 The result is a dictionary mapping circle names to SymPy ``Circle`` objects. 340 We can verify that the circles have been imported correctly: 341 342 >>> circle['C1'] 343 Circle(Point(2, 3), 3.0) 344 >>> circle['C2'] 345 Circle(Point(1, 1), 4.0) 346 347 Finally we can find the exact intersection points with SymPy: 348 349 >>> circle['C1'].intersection(circle['C2']) 350 [Point(29/17 + 64564173230121*sqrt(17)/100000000000000, 351 80705216537651*sqrt(17)/500000000000000 + 31/17), 352 Point(64564173230121*sqrt(17)/100000000000000 + 29/17, 353 80705216537651*sqrt(17)/500000000000000 + 31/17)] 
trunk/docs/contents/pgdb/adaptation.rst
r799 r807 268 268 269 269 >>> pgdb.set_typecast('jsonb', cast_json) 270 271 As one last example, let us try to typecast the geometric data type ``circle`` 272 of PostgreSQL into a `SymPy <http://www.sympy.org>`_ ``Circle`` object. Let's 273 assume we have created and populated a table with two circles, like so: 274 275 .. codeblock:: sql 276 277 CREATE TABLE circle ( 278 name varchar(8) primary key, circle circle); 279 INSERT INTO circle VALUES ('C1', '<(2, 3), 3>'); 280 INSERT INTO circle VALUES ('C2', '<(1, 1), 4>'); 281 282 With PostgreSQL we can easily calculate that these two circles overlap:: 283 284 >>> con.cursor().execute("""SELECT c1.circle && c2.circle 285 ... FROM circle c1, circle c2 286 ... WHERE c1.name = 'C1' AND c2.name = 'C2'""").fetchone()[0] 287 True 288 289 However, calculating the intersection points between the two circles using the 290 ``#`` operator does not work (at least not as of PostgreSQL version 9.5). 291 So let' resort to SymPy to find out. To ease importing circles from 292 PostgreSQL to SymPy, we create and register the following typecast function:: 293 294 >>> from sympy import Point, Circle 295 >>> 296 >>> def cast_circle(s): 297 ... p, r = s[1:1].rsplit(',', 1) 298 ... p = p[1:1].split(',') 299 ... return Circle(Point(float(p[0]), float(p[1])), float(r)) 300 ... 301 >>> pgdb.set_typecast('circle', cast_circle) 302 303 Now we can import the circles in the table into Python quite easily:: 304 305 >>> circle = {c.name: c.circle for c in con.cursor().execute( 306 ... "SELECT * FROM circle").fetchall()} 307 308 The result is a dictionary mapping circle names to SymPy ``Circle`` objects. 309 We can verify that the circles have been imported correctly: 310 311 >>> circle 312 {'C1': Circle(Point(2, 3), 3.0), 313 'C2': Circle(Point(1, 1), 4.0)} 314 315 Finally we can find the exact intersection points with SymPy: 316 317 >>> circle['C1'].intersection(circle['C2']) 318 [Point(29/17 + 64564173230121*sqrt(17)/100000000000000, 319 80705216537651*sqrt(17)/500000000000000 + 31/17), 320 Point(64564173230121*sqrt(17)/100000000000000 + 29/17, 321 80705216537651*sqrt(17)/500000000000000 + 31/17)]
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