ENH: Add papersizes (#800)

PyPDF2/__init__.py

@@ -1,11 +1,14 @@
-from ._version import __version__
-from .merger import PdfFileMerger
-from .pagerange import PageRange, parse_filename_page_ranges
-from .pdf import PdfFileReader, PdfFileWriter
+from PyPDF2._version import __version__
+from PyPDF2.merger import PdfFileMerger
+from PyPDF2.pagerange import PageRange, parse_filename_page_ranges
+from PyPDF2.papersizes import PaperSize
+from PyPDF2.pdf import PdfFileReader, PdfFileWriter
+from PyPDF2 import pdf
 
 __all__ = [
     "__version__",
     "PageRange",
+    "PaperSize",
     "parse_filename_page_ranges",
     "pdf",
     "PdfFileMerger",

PyPDF2/papersizes.py

@@ -0,0 +1,48 @@
+"""Helper to get paper sizes."""
+
+from collections import namedtuple
+
+Dimensions = namedtuple("Dimensions", ["width", "height"])
+
+
+class PaperSize(object):
+    """(width, height) of the paper in portrait mode in pixels at 72 ppi."""
+
+    # Notes how to calculate it:
+    # 1. Get the size of the paper in mm
+    # 2. Convert it to inches (25.4 millimeters are equal to 1 inches)
+    # 3. Convert it to pixels ad 72dpi (1 inch is equal to 72 pixels)
+
+    # All Din-A paper sizes follow this pattern:
+    # 2xA(n-1) = A(n)
+    # So the height of the next bigger one is the width of the smaller one
+    # The ratio is always approximately the ratio 1:2**0.5
+    # Additionally, A0 is defined to have an area of 1 m**2
+    # Be aware of rounding issues!
+    A0 = Dimensions(2384, 3370)  # 841mm x 1189mm
+    A1 = Dimensions(1684, 2384)
+    A2 = Dimensions(1191, 1684)
+    A3 = Dimensions(842, 1191)
+    A4 = Dimensions(
+        595, 842
+    )  # Printer paper, documents - this is by far the most common
+    A5 = Dimensions(420, 595)  # Paperback books
+    A6 = Dimensions(298, 420)  # Post cards
+    A7 = Dimensions(210, 298)
+    A8 = Dimensions(147, 210)
+
+    # Envelopes
+    C4 = Dimensions(649, 918)
+
+
+_din_a = [
+    PaperSize.A0,
+    PaperSize.A1,
+    PaperSize.A2,
+    PaperSize.A3,
+    PaperSize.A4,
+    PaperSize.A5,
+    PaperSize.A6,
+    PaperSize.A7,
+    PaperSize.A8,
+]

Tests/test_papersizes.py

@@ -0,0 +1,30 @@
+from PyPDF2 import papersizes
+import pytest
+
+
+def test_din_a0():
+    dim = papersizes.PaperSize.A0
+    area_square_pixels = float(dim.width) * dim.height
+
+    # 72 pixels is 1 inch
+    area_square_inch = area_square_pixels / 72**2
+
+    # 25.4 millimeter is equal to 1 inches
+    area_square_mm = area_square_inch * (25.4)**2
+    assert abs(area_square_mm - 999949) < 100
+    conversion_factor = 72 / 25.4
+    assert (dim.width - 841 * conversion_factor) < 1
+    assert (dim.width - 1189 * conversion_factor) < 1
+
+
+
+@pytest.mark.parametrize("dimensions", papersizes._din_a)
+def test_din_a_ratio(dimensions):
+    assert abs(dimensions.height - dimensions.width * 2**0.5) <= 2.5
+
+
+@pytest.mark.parametrize(
+    "dimensions_a, dimensions_b", list(zip(papersizes._din_a, papersizes._din_a[1:]))
+)
+def test_din_a_doubling(dimensions_a, dimensions_b):
+    assert abs(dimensions_a.height - 2 * dimensions_b.width) <= 4