Python-pptx: Moving and resizing placeholders - What is the right order and why?

Question

I am using Python-pptx to automatically fill a company templace pptx with pictures. The template consists of a title and subtitle line, 2 image placeholders at the top of the slide body and 2 text placeholders below.

Using the following code

prs = pptx.Presentation(r'template.pptx')
comp_slide_layout = prs.slide_layouts[SLD_LAYOUT_IMGCOMP]

slide = prs.slides.add_slide(comp_slide_layout)
title = slide.shapes.title
subtitle = slide.placeholders[59]

title.text = md_title
subtitle.text = md_subtitle

cap1_ph = slide.placeholders[74]
cap2_ph = slide.placeholders[71]

cap1_ph.text = 'placeholder 71'
cap2_ph.text = 'placeholder 74'

pic1_ph = slide.placeholders[72]
pic2_ph = slide.placeholders[73]

I get a decent result.

For customizing the template file, I want to have smaller text placeholders and larger image placeholders.

Thus, I try to change the size of the placeholders:

# reading the horizontal and vertical anchor points of the img's
left1 = pic1_ph.left 
left2 = pic2_ph.left
topimg = pic1_ph.top

# setting new top anchor for captions
topcap = cap1_ph.top = cap2_ph.top = topimg + pptx.util.Cm(11)

# changing sizes of placeholders
pic1_ph.height = pic2_ph.height = pptx.util.Cm(10)
cap1_ph.left = pic1_ph.left
cap2_ph.left = pic2_ph.left
cap1_ph.height = cap2_ph.height = pptx.util.Cm(2)

Now, the text placeholders should be 2 cm high and shifted downwards, where the width of both picture and text placeholders is changed nowhere. Still, the result is as follows:

Both image (marked in red) and text placeholders (marked in blue) have their widths squeezed to zero, where the position of the text placeholders seems to be correct, but the picture placeholders are also shifted to the left edge of the slide.

If the codelines regarding the left anchors are changed to

cap1_ph.left = pic1_ph.left = left1
cap2_ph.left = pic2_ph.left = left2

the result is this:

Although the only thing that was changed is the left anchor for captions and pictures, now the top anchors and the widths of the picture placeholders are changed.

To save you some time, I will skip all the other steps of trial and error and just say, with reading the widths after the "naming the placeholders"-block and re-setting widths and top anchors of the pictures at the very end, it works.

The final code is then:

prs = pptx.Presentation(r'template.pptx')
comp_slide_layout = prs.slide_layouts[SLD_LAYOUT_IMGCOMP]

slide = prs.slides.add_slide(comp_slide_layout)
title = slide.shapes.title
subtitle = slide.placeholders[59]

title.text = md_title
subtitle.text = md_subtitle

cap1_ph = slide.placeholders[74]
cap2_ph = slide.placeholders[71]

cap1_ph.text = 'placeholder 71'
cap2_ph.text = 'placeholder 74'

pic1_ph = slide.placeholders[72]
pic2_ph = slide.placeholders[73]

width = pic1_ph.width

# reading the horizontal and vertical anchor points of the img's
left1 = pic1_ph.left 
left2 = pic2_ph.left
topimg = pic1_ph.top

# setting new top anchor for captions
topcap = cap1_ph.top = cap2_ph.top = topimg + pptx.util.Cm(11) #11*360000

# changing sizes of placeholders
pic1_ph.height = pic2_ph.height = pptx.util.Cm(10) # 10cm (360 000 emu = 1 cm)
cap1_ph.left = pic1_ph.left = left1
cap2_ph.left = pic2_ph.left = left2
cap1_ph.height = cap2_ph.height = pptx.util.Cm(2)#860983

pic1_ph.width = pic2_ph.width = cap1_ph.width = cap2_ph.width = width
pic1_ph.top = pic2_ph.top = topimg

And the result looks like this:

However, it seems very random to me, which and how many parameters are changed unintendedly when I want to change exactly one. Moreover, there is a right order of changing parameters, but also this seems completely random to me.

Still I would like to understand, why the Python-pptx package is working like that, specifically addressing the following questions:

1. Why there are parameters that change unintendedly when actually chaning other parameters?
2. Is there a general "correct" order of changing parameters?
3. If so, what is this order and why is it like this?

Can anyone enlighten me (and others)? Thank you!

---

EDIT:

There are even more strange things happening, as I continue to play around. I will list more examples of odd behaviour as I encounter them, hoping that someone can explain, why this is happening.

1) Using

cap1_ph.height = cap2_ph.height = pptx.util.Cm(2)

leads to only cap2 having a height of 2 cm, where cap 1 has zero height.

Using instead

cap1_ph.height = pptx.util.Cm(2)
cap2_ph.height = pptx.util.Cm(2)

produces the intended result.

2)

When I change the dimension and position of the picture placeholder and insert pictures, the picture is cropped. I have to change the dimension of the picture again after pic1_ph.insert_picture() to get a properly displayed figure. (If I don't change the placeholder dimension before the insert_picture, the figure is cropped and stretched.)